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         Forces of Nature
         an you explain the difference between centrifugal and centripetal
         force? Are you well versed in entropy and its effects? If your
         knowledge needs a bit of dusting off, the Forces of Nature eBook
from Motion System Design is a thorough yet entertaining look at some of
the most important physical forces in the universe. From microgravity to
inertia, this handy eBook is designed to refresh you on the following topics:
Entropy, inertia, microgravity, and the four universal forces — gravity,
electromagnetism, and the weak and strong nuclear forces.

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                   Forces of Nature                                                                                   Automation Direct:
                                                                                                                          Investing in our educational future
                               table of contents

                                              4                                                                       A
                                                      The four forces: Four fundamental forces appear to be                   utomationDirect announced the opening of the new
                                                                                                                              AutomationDirect Automation Laboratory on the
                                                      holding the Universe together: the electromagnetic force,
                                                                                                                              campus of Northern Illinois University (NIU). As the field
                                                      two nuclear forces, and gravity. They’re all caused by          of automation constantly changes, educators need to maintain
                                                                                                                      pace with these changes in the classroom to produce well-
                                              matter’s absorption or emission of energy-carrying particles called     qualified graduates. Teaching advanced hands-on concepts
                                                                                                                      requires complex laboratories and equipment. AutomationDirect
                                                                                                                      and NIU believe one solution to this complex situation is through
                                                                                                                      innovative Industry/Education partnerships.

                                                                                                                         During the past few years, AutomationDirect has provided             AutomationDirect supports education
                                                      Physical constants: These are numbers, so important             control products, including PLCs, to NIU’s Department of             nationwide primarily through product
                                                                                                                      Technology. AutomationDirect has now cooperated with                 donations to colleges, universities,
                                                      to engineering work, that you once committed to memory.         the Department to help fund a well-equipped automation               trade-schools, and continuing education
                                                                                                                      laboratory. In return, the Department named the lab the              programs. They are also actively involved
                                                      Let’s review.
                                                                                                                      “AutomationDirect Automation Laboratory” and created the             with education at the primary school level
                                                                                                                      AutomationDirect Professorship. The laboratory is designed           with local robotics team sponsorships
                                                                                                                      to support the Departmental Programmable Logic Controller            and as a kit-of-parts sponsor of the FIRST
                                                                                                                      (PLC) course and the Automation course. The Programmable             robotics program.

                                                                                                                      Logic Controller (PLC) course teaches students basic and                AutomationDirect sells thousands
                                                             Entropy: From the erosion of a grain
                                                                                                                      advanced concepts and applications for PLC programming. In           of feature-packed products through
                                                             of sand off a cliff to the unbounded expansion of        addition, the students learn to integrate various components         its Online Automation Superstore and
                                                                                                                      with the PLC, such as sensors, switches, and output devices.         2,200-page catalog. The company offers
                                                             the Universe, Nature tends towards disorder. This           As a final project, student teams design automation systems       its customers high-quality automation
                                                                                                                      which sort parts, using the integration of vision, PLCs, sensors,    products at prices well below the industry
                                              tendency has a name — entropy. The phenomenon has specific                                                                                   average, 24-hour order entry with same-
                                                                                                                      and pneumatics. This level of experimental interaction produces
                                              scientific meaning.                                                     Manufacturing Engineering Technology graduates who excel in          day shipping on in-stock items, the best
                                                                                                                      industry.                                                            documentation in the industry, and award-
 This arrangement also has relatively low                                                                                With AutomationDirect’s support, the school will have a           wining sales and technical support.

     multiplicity — as there not many
permutations for which only two molecules
                                                                                                                      very strong laboratory facility in which the next generation of
                                                            Microgravity: To peel away the cloak of gravity and
 are found in the left side of the chamber.                                                                           manufacturing engineering technologists will be educated.                               Visit
                                                            isolate some of the more fundamental properties,             The Department of Technology has been the cornerstone of                                    for more information.
                                                                                                                      technical education at Northern Illinois University for over 80
              =                                             forces, and processes of nature, scientists create a      years. The department has evolved from vocational education
                                                                                                                      to an organization supporting the needs of industry and
                                      =       condition called “microgravity.”                                        the region. A component of the College of Engineering and
                                                                                                                      Engineering Technology, with approximately 1,500 students,

               =                                             Inertia: Matter tends to resist change. Around
                                                                                                                      the Technology Department has over 400 students participating
                                                                                                                      in Electrical Engineering Technology (EET), Manufacturing
                                                                                                                      Engineering Technology (MET), and Industrial Management
                                                             moving masses, inertia acts like a stabilizer, pushing
                                                                                                                      and Technology programs. The department is accredited by
                                                             against any accelerating or decelerating force.          the National Association of Industrial Technologists (NAIT)
                                                                                                                      and the Accreditation Board for Engineering and Technology

          2       FORCES of NATURE eBook                              Sponsored by AUTOMATIONDIRECT                                        AUTOMATIONDIRECT.COM • FORCES of NATURE eBook                                               3
The four forces                                                     Bosons are
                                                             particles that carry force.
                                                                                                                                                                   Fermions are particles
                                                                                                                                                                    that make up matter.



                                                                               i to
           our fundamental forces


           appear to be holding                                                                                                                 nd

           the universe together:

 the electromagnetic force, two

 nuclear    forces,   and     gravity.                                                       ns
 They’re all caused by matter’s                                                       ot                                             Quarks
                                                                                 Ph                                 always collect into Hadrons.
 absorption or emission of energy-                                                                                     There are two kinds:

 carrying particles called bosons.                                                                                     Mesons         Baryons

 For this reason, all might be more                                                                          ng
 aptly called interactions.                                                                                        nu                                              Leptons:
                                                                                                                           c le a r                                Electrons
   Most physicists today believe                                                                                                                                    Muons
                                                 The Grand Unification Theory says that
 the four forces are actually just          because the strong nuclear gets weaker while                                                                           Neutrinos
 aspects of one Force of the                 the electromagnetic and weak nuclear forces
                                         get stronger at high energies, they could eventually                                                             W
 universe. That’s why many are                be considered one and the same. However,                                                                        ea
                                                  creating an energy environment high                                                                              k
 working on a Unified Theory
                                           enough to verify this is currently impossible.                                                                              nuc
 to explain how they all relate.
                                                                                                                                                                               le a r
 In fact, three of the force types

 have already been linked with

 experiments.    Gravity      —   the
 renegade of the bunch — is the                                                        ag
                                                                                                        ic                                               G rav
 only one that remains unlinked.                                                                                                                                   ita tio n a l

  4    FORCES of NATURE eBook               Sponsored by AUTOMATIONDIRECT                                                             AUTOMATIONDIRECT.COM • FORCES of NATURE eBook         5
    Strong nuclearctive rce
          Direction: Always
     Affects: Quarks, binding them into 1)
 unstable pairings of quarks and antiquark
to form mesons or more commonly 2)        stable
 quark triplets called baryons. Protons and
      neutrons are two kinds of baryons.
               Carried by: Gluons
 Claim to fame: This force groups quarks
   into neutrally “colored” particles. In add ition,
                                                  e                            Weak nu
    the gluons (through and on which this forc                              Direction  c                 lear forc
      acts) also stay grouped in these special                               sense, th
                                                                                           : T h ou g h
                                                                                                          not direc           e
                                                                                                                    tional in th
     arrangements. Atomic bombs and nuclear                                                e force a
                                                                                                        ffects ma                e
                                                   e                        in two dis                             tter or an usual
    reactors cash in on the strong nuclear forc                                           tinc
                                                                            why the u t, nonsymmetrica                        timatter
 stored in atomic nuclei by knocking them                                                 niverse c                    l
                                                                                                        ontains m ways. This is
                Relative strength: 1                                                                                ore matte
                                                                           Affects:                 antimatte                   r than
                                                                                        Particles               r.
                                                                            multiple th                with a sp
                                                                                            ereof —              in of 1
                                                                                                         in other w /2 or some
                                                                                               particles             ords, ma
                                                                        Carried b                            only.              tter
                                                                                     y: Ver y h
                                                                          (which ac                 eavy W+, -
                                                                                       t just like              W , and Z
                       f e ce
                                                                                                                            b oso ns
                                                                         Claim to                    photons
                                                                                      fame: Un                  at h
                                                                                                     like the o igh energy).
        atio                                                            weak nuc
Graviction: Always at nything with
                         ti                                                         lear force                  ther force
                                                                      particles                     a                       s, th
      t                                                                           into comp ctually changes m e
        Dire                 cts a                                                                letely diffe               a
                   vity affe               words,
                                                    all                allowing
                                                                                  all                          rent parti tter
         ts: Gra gy — in other                                        antipartic leptons and quark                        cles by
 Affec             ner                                                           les                            s and
          nd/or e articles.                             d            energy, m to interchange
m as s a              p                       obser ve                           ass, elec
                           ns —     not yet exist.                     charge, a               tric
                 Gravito y believed to                                             nd flavor.
Carr   ied by:               ll                     d the             Relative
                    genera                 nsidere r                             strength
   in a  lab, but Alternately co                     ,o                                         :
                      :                      ce field                           10 -5
 Claim     to fame ce-time, a for y far the
                        a              vity is b
              re of sp                                    e
    cur vatu exchange, gra d yet (becaus
              on                     , an
    a gravit all the forces ) it’s the easie
             t  of                   nc e
   weakes r infinite dista                 bser ve.
    it ac ts ove         o sc o p ically o 10-39
               to macr strength: 6×
                 Re  lative

                                                                       Direction: Att
                                                                                           etic force
                                                                                          ractive or repu
                                                                     Affects: Elect                       lsive
                                                           Carried by: V              rically charged
                                                                         irtual photons                 particles
                                                         they’re imagin                     — called virtua
                                                                        ar y, but becaus                     l not because
                                                                 charged partic             e they’re exch
                                                                                  les (like electr          anged betwee
                                                                                                   ons) so quickl            n
                                                                Claim to fame:                                     y.
                                                               the basis of al                      ly useful, this fo
                                                                               l electrical desi                       rce is
                                                                 released when                    gns today. The
                                                                                   electrons chan                   photons
                                                                                                     ge shells also
                                                                                       human sight.                   allow
                                                                               Relative stren
                                                                                                 gth: 7×10 -3

6       FORCES of NATURE eBook                                    Sponsored by AUTOMATIONDIRECT                                          AUTOMATIONDIRECT.COM • FORCES of NATURE eBook   7
                              Up                                                                                                Planck’s constant

       Physical constants
             Gravitational acceleration                                                 Avogadro’s number                                              Gas constant

                                                                      Electron charge

                                                                                                                              Gravitational constant

    Particle mass
                                                                                              Boltzmann’s constant
                                                                                                                                                                      Bohr radius

                                              Speed of light

                                                                                              Also see page 13 about the
                                                                                         Boltzmann relationship to entropy.

8   FORCES of NATURE eBook                Sponsored by AUTOMATIONDIRECT                                     AUTOMATIONDIRECT.COM • FORCES of NATURE eBook                           9
                                                                                                                                                                                                                Tomato, tomaaato
                                                                                                                                                                                                                   Just as light can be defined as a particle
                                                                                                                                                                                                                or as a wave, so too can entropy be scientifi-
                                                                                                                                                                                                                cally described in two different ways. The first
                                                                                                                                                                                                                way is in terms of heat — a view developed
                                                                                                                                                                                                                by Frenchman Nicolas Léonard Sadi Carnot
                                                                                                                                                                                                                in 1824.
                                                                                                                                                                                                                   Assume that we have a two-chambered in-

                                                                                                                                                                                                                sulated tank; ideal gas fills one half. If we re-

                                                                                                                                                                                   Courtesy Great War Primary
                                                                                                                                                                                       Document —
                                                                                                                                                                                                                move the barrier between the two chambers,
                                                                                                                                                                                                                our ideal gas will expand to fill both evenly.
                                                                                                                                                                                                                The increase in entropy for an isothermal pro-
                                                                                                                                                                                                                cess is:
                                                                                                                                                                                                                            S = Sf    Si =
                                                                                                                                                                                                                   where Si = Initial entropy

                                                                                                                                                                                                                           S f = Final entropy
                                                                                                                                                                                                                            Q = Heat to or from system
                                                                                                                                                                                                                                      V final
                                                                                                                                                                                                                              = nRT ln

                                          Call me irreversible                                                                                                                                                                        Vinitial
                                                                                                                                       rom the erosion of a grain of sand off a
                                            Shattering a glass, scrambling an egg, sending an                                                                                                                               T = Temperature, kelvins
                                          unfortunate reply-all email — what is it about these
                                                                                                                                       cliff to the unbounded expansion of the                                     Measured in Joules per Kelvin, this entropy
                                          actions that makes them impossible to reverse? Well,                                                                                                                  change does not need to be expressed as the
Courtesy Kyle May

                                          that’s complicated; the orderly reassembly of atoms or                                       Universe, Nature tends towards disorder.                                 integral, as it is equal to that of a reversible
                                          restoring of situations breaks a few rules of our universe,                                                                                                           process with the same initial and final states.
                                          not the least of which have to do with directionality of                           This tendency has a name — entropy. The
                                            Suffice it to say that irreversible events are in
                                                                                                                             phenomenon has specific scientific meaning, but
                                          effectively closed systems, and it’s a law of physics                              in everyday popular culture, is described succinctly
                                          (the Second Law of Thermodynamics, to be exact)
                                          that entropy, sometimes also called the Arrow of Time,                             and wittily by the infamous Murphy’s Law — What can
                                          always increases in such situations. In theory, reversible
                                          processes do exist — for example, squeezing air from                               go wrong, will go wrong.
                                          one end of a balloon to another — but in reality, even
                                          these create friction and other nonreversible results.

                                                                                                                                Murphy’s Law:
                                                                             What’s more, the entropy of two joined sys-
                                                                                                                                Correlates and functions
                                                                             tems is greater than the pair’s two entropies
                                                                                                                                  If left alone, things tend to go from bad to worse. • If a series of events can go wrong, they
                                                                             if they remain separated — as the removal of
                                                                                                                                will do so in the worst possible sequence. • Nothing is as easy at it looks. • If everything
                                                                             a boundary to allow mixing makes for a less
                                                                                                                                seems to be going well, you have obviously overlooked something.
                                                                             organized situation.

                                        Entropy and black holes
                                          Mexican-born Israeli Jacob Beckenstein proposed in 1972
                                        that even black holes obey the Second Law of Thermodynamics,
                                                                                                                                                                        Fan of the flames: Sadi Carnot wrote his short book,
                                        and have a definable amount of entropy. This implies that black
                                                                                                                                                                        Reflections on the Motive Power of Fire, in 1824. It
                                        holes also have a temperature, and emit the associated radiation
                                                                                                                                                                        details how motion can be generated from the “fall” of
                                        and particles — something that Stephen Hawking reluctantly
                                                                                                                                                                        heat from one object to another colder body. It also first
                                        confirmed a year later. Today it is generally accepted that a
                                                                                                                                                                        outlined the Second Law of Thermodynamics.
                                        black hole’s event-horizon area is a measure of its entropy.

                    10   FORCES of NATURE eBook            Sponsored by AUTOMATIONDIRECT                                                             AUTOMATIONDIRECT.COM • FORCES of NATURE eBook                                                         11
                                                                                                                                                                                           Common — tendency to entropy

              s a measurement of a system’s                                                                         Odds are even                                                             Configurations with higher multiplicity
                                                                                                                       The first way to describe entropy is in terms of thermodynamics     and probability of occuring (as the below
              randomness, entropy has                                                                               or heat. The second way to describe entropy is with statistics.        arrangements, in which the molecules
              specific scientific meaning.                                                                                                                                                 are spread equally over chamber halves,
                                                                                                                                                                                           for example) also have higher entropy, as
                                                                                                                        Assume that we now have a single-chamber insulated                there are more channels for free energy.
                                                                                                                    tank that contains one dozen identical molecules of an ideal
                                                                                                                       At any instant, each of our molecules has a 50/50 chance
                                                                                                                    of being on one half of the chamber. However, the position
  Spread, sprinkled, or expanded systems in general possess                                                         permutations for which all 12 of the molecules are on one side
higher relative entropy: Mustard in a neat line on a hotdog is                                                      at once are miniscule; rather, probability is highest that at any
less entropic than mustard splattered on a shirt; similarly, the                                                    given moment, the molecules will be spread evenly throughout
carbon in a diamond is more orderly and less entropic than                                                          the chamber. This unfurled arrangement has higher entropy
                                                                             This arrangement (with all 12 of       and is expressed by a relationship developed by Austrian Louis
that in graphite.
                                                                           our molecules collected in one side      Boltzmann:
  Speaking of allotropes, if systems tend towards increasing              of the tank) is possible but unlikely —
entropy, then why do materials freeze into orderly crystal                        and has low multiplicity.
                                                                                                                                S = k lnW
lattices? Even here, the Second Law of Thermodynamics                                                                     where k = Boltzmann contant
                                                                                                                                                                    Boltzmann’s constant relates the macroscopic
holds. Entropy coexists with forces that drive atoms and                                                                                Gas constant               ideal gas law (of pressure, mass, volume, and
molecules to lower energy states; even where crystals                                                                             =
                                                                                                                                    Avogadro's number
                                                                                                                                                                temperature) to the microscopic physics of our 12
themselves are lower in entropy and energy, they form in                                                                                                          identical molecules. Multiplicity is the number of
non-isolated systems that interact with the environment, its                                                                          8.314                   ways that the molecules can rearrange to make one
kinetics, and greater energy.                                                                                                     =         K mol                           essentially identical situation.
                                                                                                                                    6.022 10 23
                                                                                                                                  = 1.38 10 J/K

                       The densest lattices (as the face-centered cubic here) are most efficient                              W = Multiplicity of the situation
                            for packing spherical atoms. Researchers at the University of
                        Pennsylvania led by Randall D. Kamien have shown that though they’re                                     Note that this statistical definition is related to the
                       more unique, systems of oddly shaped molecules also freeze into efficient                              thermal definition, as molecules must have the energy to
                            and predictable lattices for maximum overall system entropy.                                      spread through the chamber — and any resulting entropy
                                                                                                                              results from their interactions.

                                                                                                                                                 Two on the left
                                                                                          Courtesy Jon Sullivan

                                                                                                                                                                                           W =
                                                                                                                                                                                                 nL! nR!
                                              In a paper published in the American Journal of Physics,                                                                                              Total molecules (factorial)
                                               Daniel F. Styer proposes that the hard association of                                                                                            In left (factorial) In right (factorial)
                                             entropy with notions of disorder is troublesome, and that
                                            the scientific definitions deserve more distinction. He also                                                                                       =
                                                       proposes the simile entropy as freedom.                                                                                                   2! 10!
                                                 In this case, two suddenly joined (and possibly less                                         This arrangement also has relatively low            479,001,600
                                                                                                                                                  multiplicity — as there not many             =
                                             “organized”) systems might also be called less bounded.                                         permutations for which only two molecules
                                                                                                                                                                                                 2 3,628,800
                                                                                                                                              are found in the left side of the chamber.       = 66

  12    FORCES of NATURE eBook                       Sponsored by AUTOMATIONDIRECT                                                         AUTOMATIONDIRECT.COM • FORCES of NATURE eBook                                         13
              v i y
 M icr o g ra    t                                                                                                                Einstein on trial
                                                                                                                                     The Satellite Test of the Equivalence Principle
                                                                                                                                  (STEP) will carry concentric test masses

                 ravity, the attractive force between two masses, keeps us oriented in                                            into orbit to test a fundamental assumption
                                                                                                                                  underlying Einstein’s theory of general relativity:
                 more ways than one. Without gravitational influences, we’d barely be
                                                                                                                                  that gravitational mass is equivalent to inertial
                 able to recognize our world, let alone function in it.                                                           mass. During the experiment, four pairs of test
                                                                                                                                  masses will be falling, and their accelerations
     Sometimes gravity gets in the way, however, blinding us to how things actually
                                                                                                                                  will be measured a million times more accurately
     work. Even our mechanical aptitude possesses a certain near-sightedness; we                                                  than in any ground-based test.
     understand things based on the way they behave in the Earth’s gravitational

     field, and that, only at the surface.
                                                                                               Newton’s apple

     To peel away the cloak of gravity and isolate some of the more                              If you drop an apple on Earth, it falls to the ground at a rate of 1g. If an astronaut
     fundamental properties, forces, and processes of nature, scientists create a              drops an apple on the space shuttle, it falls too; it just doesn’t look like it’s falling.
                                                                                               That’s because the apple, astronaut, and shuttle are all falling together. But they’re
     condition called “microgravity.”                                                          not falling toward the earth, they’re falling around it — in orbit.

                                                                                                 Newton developed a “thought experiment” to demonstrate this concept: Imagine
     Microgravity is relative rather than absolute. It’s created by free-fall; the entire           placing a cannon atop a mountain. Once fired, a cannonball immediately
     experiment accelerates at 1g, nullifying the effect of Earth’s gravity. Otherwise,                       begins falling toward the Earth. The greater the
                                                                                                                speed, the farther it travels. If fired with enough
     you’d have to travel almost 17 times farther than the Moon, or 6.37 million km, to                          speed, the cannonball could circle, or orbit, the
     reach a point in space where gravity is one-millionth of that on Earth’s surface.                           Earth in a state of continuous free-fall.

     By conducting experiments in microgravity, researchers hope to uncover new

     information previously masked by the effects of Earth’s gravitational field.

     Primary areas of investigation range from fundamental and fluid physics to
                                                                                            Look out below
     biotechnology, combustion science, and materials science. NASA even hopes to
                                                                                              The STEP experiment is a 21st-century version of a test Galileo
     test Einstein’s theory of general relativity and Newton’s law of gravity.              performed by dropping a cannonball and a musket ball from atop the
                                                                                            Tower of Pisa to compare their accelerations.

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                                                                                                               (Photos courtesy Dr. Jean-Pierre Wery and Eli Lilly and Co.)
                                                                                                                                                                              Breaking the law of gravity
                                                                                                                                                                                                                              Space station
                                                                                                                                                                                                                                     The International Space Station is a semi-
                                                                                                                                                                                                                                  permanent facility that lets scientists conduct
No more tears                          Bubble bath                        Crystal clear                                                                                                                                           research in microgravity over a period of months,
   The familiar teardrop-shaped           When a liquid is heated from        Proteins grown in microgravity                                                                                                                      without having to return the test setup to Earth
yellow candle flame is the             the bottom, small bubbles of       are many times longer than                                                                                                                              after every experiment.
result of gravity-driven buoyant       hot gas form and are carried       those grown on Earth. The
convection, which carries soot         upward     by     gravity-driven   raf kinase crystals, below, the                                                                                                                         Space shuttle
to the flame’s tip. In microgravity,   convective flows. In the same      largest ever produced, were                                                                                                                                By maintaining a consistent orbit, a space
where convective flows are             setup in microgravity, the         grown during the second U.S.                                                                                                                            shuttle can provide up to 17 days of high quality
absent, the flame is spherical,        heated gas bubbles grow            Microgravity Laboratory (USML-                                                                                                                          microgravity conditions. Shuttles can accommodate
soot-free, and blue. The study of      larger and remain attached to      2) mission in November 1995. In                                                                                                                         a wide range of equipment, functioning as a
combustion is vitally important        the bottom for a longer period     general, the larger the crystal,                                                                                                                        laboratory in which scientists can conduct long-
today as it accounts for 85% of        of time because of the lack of     the more structural information                                                                                                                         term investigations.
the world’s energy use.                convection and buoyancy.           it reveals.
                                                                                                                                                                                                                                  Sounding rockets
                                                                                                                                                                                                                                     An experiment placed in a rocket and launched
                                                                                                                                                                                                                                  along a parabolic trajectory experiences
                                                                                                                                                                                                                                  microgravity for several minutes prior to re-entering
                                                                                                                                                                                                                                  Earth’s atmosphere as the rocket is in free-fall.

                                                                                                                                                                                                                                  Reduced gravity aircraft
                                                                                                                                                                                                                                     Flying a plane over a parabolic arc can
                                                                                                                                                                                                                                  produce microgravity conditions lasting 20
                                                                                                                                                                                                                                  to 25 sec. NASA uses a modified KC-135
                                                                                                                                                                                                                                  four-engine jet transport for such tests,
                                                                                                                                                                                                                                  performing as many as 40 trajectories
                                                                                                                                                                                                                                  during a 2-hr flight.
                                 Level best
                                     A gallium-doped germanium crystal grown aboard a
                                                                                                                                                                                 Drop tower
                                                                                                                                                                                   Drop towers are long vertical shafts where test
                                   sounding rocket demonstrates the effect of gravity on                                                                                         objects achieve microgravity through free-fall.
                                   material properties. The random striations visible in the                                                                                     Glenn Research Center in Cleveland operates two
                                   bottom portion, which solidified under gravity, are an                                                                                        drop towers — one above ground (24 m tall), one
                                                                                                                                                                                 underground (132 m deep). Here, scientists can
                                   indication of variations in gallium concentration. The top                                                                                    perform microgravity experiments lasting 2.2 to 5.2
                                   portion grew in microgravity, where the lack of buoyancy-                                                                                     sec.
                                  induced convection allowed the mixture to remain uniform
                                 as the melt solidified.
                                                                                                                                                                              Thanks to NASA for technical support and graphics, and Steve Lambing for technical input.

     16      FORCES of NATURE eBook                      Sponsored by AUTOMATIONDIRECT                                                                                                          AUTOMATIONDIRECT.COM • FORCES of NATURE eBook                                    17
       Inertia                                                                                 Point of view
                                                                                               as just zero-motion


                                                                                               leads to the concept of
     Stuck in its ways                                                                         nonaccelerating inertial                                   =
                                                                                               frames: As long as
     Inertia is matter’s tendency to resist                                                    frames move uniformly
     change. Around moving masses it                                                           they’re like stationary
                                                                                               references, so masses
     acts like a stabilizer, pushing against
                                                                                               in these frames all observe the same laws of physics. (On
     any accelerating or decelerating force.                                                   the other hand, accelerating objects encounter fictitious
                                                                                               forces such as the Coriolis effect.)
     Galileo first defined the principle of
                                                                                                 Ernst Mach first proposed that any system’s inertia is the
     inertia in the 16th century; it states that                                               result of its interaction with the rest of the universe; Albert
     every object persists in its state of rest                                                Einstein later vouched for this statement’s refutation of
                                                                                               absolute reference frames — and differentiating rest and
     or uniform motion unless compelled                                                        motion — by reasserting the equivalence of all inertial
     by outside forces. Isaac Newton later                                                     frames.
     adopted this principle as his First Law

     of Motion.

                                                                                                                      Related concept: Momentum
                                                                                                                        Inertia depends only on mass, but momentum
                                                                                                                      depends on mass and speed. The same amount
                    z                                                                                                 of momentum might be found in a faster/small
                                                                                                                      mass as in a slower/large mass. Similarly, when
                                     Take it for a spin                                                               a twirling skater draws her arms inward, her
                                       Also called rotational inertia, an object’s moment                             moment of inertia decreases so her angular
                                     of inertia is its tendency to continue rotating free of                          velocity must increase. That’s because angular
          x                   y      angular acceleration unless an external torque is                                momentum is also conserved, so the product
                                     applied. The strength of this tendency is measured in                            of angular velocity times moment of inertia is
                                     kg-m2 and varies with the object’s shape.                                        constant.

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                                              Inertial = gravitational
                                                Newton’s Second Law of Motion
                                              assumes mass is an object’s resistance
                                              to acceleration, but Newton’s Law of
                             mboat . mearth
                     F = G                    Universal Gravitation assumes mass
                                              measures gravitational attraction. The
                                              dichotomy is resolved in the Principle
                                              of Equivalence, which states there
                                              is no way to distinguish between the
                                              effects of acceleration and gravity.
                                              Indeed, some physicists discriminate
                       F = mboat . a          gravitational mass mg from inertial mass
                                              mI. However, in our universe it appears
                                              the two masses are always equal.

                                    Where does inertia come from?
                                      Truth is, no one really knows. While some
                                   scientists argue that inertia isn’t a fundamental
                                   property, others think it indeed relates to matter’s
                                   essential nature — it just hasn’t been addressed by
                                   modern theories yet.
                                      Scientists at the California Institute for Physics
                                   and Astrophysics, Palo Alto, Calif., and elsewhere
                                   are exploring inertia’s possible origins. They’re
     trying to reformulate older concepts within more sophisticated quantum field
     (QFT) and superstring theories to link inertia to the Higgs field. Why? It’s
     believed that inside this Higgs field live Higgs bosons which impart mass by
     “dragging down” all particles with which they interact. One catch: Even if the
     Higgs particle causes inertial behavior, it still won’t prove whether inertia is an
     intrinsic or extrinsic property.

20   FORCES of NATURE eBook                   Sponsored by AUTOMATIONDIRECT

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