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Energy and Forces


  • pg 1
									Concepts that drive the world
Energy: Physics
 Energy cuts
 across all
   Physics
       E = mc2
       How stars
       Electricity
       Heat
       Roller
Energy: Climate & Weather…
 Climate/Weather
        Hurricanes
       Several thousand
        atomic bombs
       Lightning
       Global warming –
        redistribution of
        energy across the
Energy: Chemistry
 Energy cuts across
 all fields
   Chemistry
       Activation energy
       Energy needed for
        a reaction to occur
       Heat and
        temperature is
       Think of Ozone...
Energy: Food
  Food
     Average American
      2700 Calories
  Note the big C!
     kilocalories
     Really means 270,000
Nutrition Labels
            Nutrition labels tell you about the
             energy content of food
            Conversions:       Fat:    9 Cal/g
                                Carbs: 4 Cal/g
                                Protein: 4 Cal/g
            This product has 72 Cal from fat, 48
             Cal from carbohydrates, and 32 Cal
             from protein
               sum is 152 Calories: compare to label
            152 Cal = 636 kJ: enough to climb
             about 1000 meters (64 kg person)
               1 Cal = 4.134 Jules
Energy and Food…
 Average American
  2700 calories
  (recommended 2200)
 7 acres to support one
 1.5 acres to support
  one African
 The world has enough
  capacity to support
  around 10 billion
So what supports me?
How did the science that we used to get to
                       the Moon evolve?
Aristotle: A long lasting paradigm
Natural motion depended on
 nature of the object.
    A rocks falls
       Smoke rises
 The falling speed of an object
 was supposed to be
 proportional to its weight
   So heavier objects fall faster
 Natural motion could be
 circular (perfect objects in
 perfect motion with no end)
   So planets orbit the Sun in perfect circles
 Aristotle: Why do things move?
 Pushing or pulling
  forces imposed
 Some motions were
  difficult to understand.
   Example: the flight of
   an arrow
      Must be some force that
       pulls it forward after it
       leaves the bow
          air rushed around
           behind the arrow and
           pushed it forward.
Other Contributions
  Aristotle also believed:
      Earth was immovable at the
       center of the Celestial Sphere,
       and -
      All other celestial bodies:
         moved with uniform speed.

         In pure circular motion

         Are "perfect" and
So how do things move anyway: The Greek
 Earth was special and at
  the center
 Series of Crystal Spheres
   The prime mover
     Outer sphere

   Just inside the prime-
    mover sphere was the
    fixed non-moving stars
   Inner spheres were the
    planets the Sun and the
The Greeks: Contributions
 One of the most important
  - and infamous -
  contributions of the
  Greeks was the geocentric
  model of the universe.
 The geocentric model
  means that the Earth,
  "geo," is at the center,
  "centric," of the universe,
  and that all other bodies
  move around it with the
  Earth at a fixed location.
Why Earth should be the center?
 This view held so much sway because of many of
  the philosophies of the ancient Greeks.
   They believed that the circle is the perfect form, and that
    the simplest model that made sense must be the correct
   since the simplest model was that the Earth stood still
    and everything moved around it, then that must also be
   After all, we can't feel the Earth moving, so why should
    be believe that it does without any extraordinary
 Aristotle: The man in science!!!!!!!!
 Aristotle was
 unquestioned for
 2000 years.
 Most thought that the Earth
  was the center of everything
  for it was in its normal state
 No one could imagine a force
  that could move it.
   It was huge!!!!!!!!!!!!!!!
COPERNICUS: A new paradigm

 Sun was center, not
 He was hesitant to
  publish because he didn't
  really believe it either.
 De Revolutionibus
  reached him on the day
  he died, May 24, 1543.
Copernicus – gradually moving
 Nicolaus Copernicus (1473-1543)
 articulated a cosmological theory
 discounting the geocentric
 Ptolemaic universe.
   did not abandon the Ptolemaic
    cosmogony all together.
   his observations resulted in a
    universe limited by the sphere of the
    fixed stars.
Copernicus not alone…
 Giordano Bruno (1548-1600) criticized
 this limited scheme.
   His theories of the infinite universe
   rejected the traditional geocentric (or
    Earth-centered) astronomy
       the sphere of the fixed stars is only a trick of
        the eye because the fixed stars are too far
        away to recognize their movement
Lets see if Bruno was right…
 Parallax, or more accurately
 motion parallax (Greek:
 παραλλαγή (parallagé) =
   apparent shift of an object
    against a background due to a
    change in observer position
   Easy to test with your finger!!
Earth and parallax
GALILEO: The emergence of a new paradigm
17th Century scientist who
 supported Copernicus.
He refuted many of
 Aristotle's ideas.
Worked on falling object
 problem - used
Galileo and myth
   Although historical
    evidence does not appear
    to support the contention,
      Galileo is alleged to
       have demonstrated by
       dropping balls of
       different weights from
       the Leaning Tower of
       Pisa that they fell with
       constant acceleration.
Quick Quiz
 Imagine the following situation:
    You have a coffee filter and a lead ball. You drop them at
     the same time which one hits first?
    Wad up the coffee filter to the same shape as the lead
     ball which one hits first?
Well…. Lets find out….
 Do some tests...

 Galileo experiment on the Moon
A tennis ball and a golf ball
dropped side-by-side in air. The
tennis ball is affected more by
the air’s resistance than the golf

The larger the object is, and the
faster it is falling, the greater
the air’s resistance to its motion,
as skydivers all know…
When most of the air is
removed from a
container, feathers and
apples fall almost side-
by-side, their speeds
changing at almost the
same rate.

If all the air was
removed, they would
accelerate downward at
exactly the same rate.
Galileo: Summary
    Knocked down Aristotle's push or pull ideas.
    Rest was not a natural state.
    The concept of inertia was introduced
    Galileo is sometimes referred to as the “father
     of experimentation.”
    Objects fall at the same rate, independent of
     weight! (acceleration, though Galileo didn’t
     know the word yet)
      Important for the navy!!!
Implications: Dropped Ball: Falling
In free fall objects accelerate
constantly toward Earth at the
rate of g . Objects moving
upward slow down until their
direction is reversed, and then
they accelerate downward.
At the top of their path the
upward speed is zero.
How long?
Only instantaneously.
A constant acceleration means
the speed is changing all the
time, so the speed only passes
through the value of zero at
the top of the path.
Tossed Ball: Falling Upward
Here two heavy balls begin
“free fall” at the same time.

The red one is dropped,
so it moves straight

The yellow ball is given some
speed in the horizontal
direction as it is released.
The horizontal lines show
that they keep pace with
each other in the vertical

They have the same
acceleration, g, downward,
and they both started with
zero speed in the downward
The yellow ball’s
horizontal speed is not
affected by gravity,
which acts only in the
vertical direction
So they hit the ground
at the same time!
Cannonballs shot horizontally with different
speeds from the ship travel different distances.
But each cannonball drops the same distance in
the same amount of time, since the vertical
acceleration is the same for each.
So no matter how fast you fire the cannonball
they are all in the air the same amount of time!!!!
Sir Isaac Newton (1642 – 1727)
Published the Principia
    Laid out his laws of motion and
     universal gravitation laws
 Newton showed that Kepler’s
  Laws are specific examples of
  broader implications of very
  general laws of motion and
 Not a very nice man
    Brilliant, knew it, didn’t
     hide it, destroyed rivals
Newton’s Three Laws
1. A body continues at rest or in uniform motion in
   a straight line unless acted on by some net
2. The acceleration of a body is inversely
   proportional to its mass, directly proportional to
   the net force, and in the same direction as the
   net force
         F = mass * acceleration
3. To every action, there is an equal and opposite
Newton’s First Law
A body at rest
tends to remain
at rest

A body in motion
tends to remain
in motion
Newton’s 1st Law: Why you wear a
Newton’s Second Law
The force exerted on an object is equal to the
 product of that object’s mass times its
The acceleration is in the same direction as the

    Force = mass x acceleration
    (force and acceleration are vectors)
Newton’s             2nd     Law
 What is Acceleration?
   Acceleration is:

    (change in velocity)/(change in
   How fast you change your velocity in a certain amount of
       Velocity is a vector – has direction and magnitude
           So change your direction   or speed you experience
Where does the bug have:
Constant velocity ?
Acceleration ?
Quiz time again
 Imagine you are driving a car. The car is moving at a
 constant velocity of 50 mph. You cover a distance of 50
 miles. What is its acceleration?
   1 m/hour2
   50 m/hour2
   2500 m/hour2
   0 m/hour2
Newton’s          3 rd   Law
 The Law of Action and Reaction:
 When any two objects interact, the force exerted by
 the first object on the second is equal and opposite to
 the force exerted by the second on the first.
   Or, for every action (force), there is an equal and
    opposite reaction; that is forces are mutual and act in
Quiz Time again: Newton’s 3rd Law
 Sometimes movies get it right
    Watch the fire extinguisher…
    Red Planet
Action-Reaction Pairs
Book pulling on Earth                     Table pushing on book

                              Physics is Fun!

Earth pulling on Book                     Book pushing on table
Newton’s Law of Gravitation
                                   G m1m2
                          F = -
   where G is the “gravitational constant”
      6.67 x 10-11 Nm2/kg2
   m1 is the mass of one body
   m2 is the mass of another body
   r is the separation between them (frequently labeled d)
      Measured from the center of each object
Inverse Square Law
Quiz Time!!
 Imagine you have two objects of equal mass (say 10
 kilograms). The two objects are setting 2 meters apart
 and you move them till they are 4 meters apart. How
 much has the gravitational force changed?
               Initial                After
                  G m1m2                   G m1m2
         Fa = -         2
                                  Fb = -
                    r                         r2

                  G m1m2                  G m1m2
         Fa = -                  Fb = -
                   2    2                   42
                  G m1m2                 G m1m2
         Fa = -                 Fb = -
                    4                       16

Double the distance decrease the force by Four
Newton Gravitation and the Earth
  From the second law of motion (F = ma), we know
   that a body of mass m subjected to the Earth's
   gravitational force of attraction F undergoes an
   acceleration at the Earth's surface of
         g = F/m.
  From the law of gravitation, this force is F =
    where m1 is the mass of the Earth, and m2 is your
     weight and r is the separation between the centers of
     the two bodies, or the Earth's radius.
Figuring out acceleration due to gravity
   Assuming that we know G, we have
    m2g = Gm1m2/r2,
      or
      g = Gm1/r2 = 9.8 m/s2
 where your mass has canceled out.
 The acceleration of the attracted body (you) does not
  depend on its own mass
    Mathematical proof that Galileo was
     Weight and Mass
   Weight is a type of force:
    It is the earth’s gravitational force on an object
   An object’s weight is proportional to its mass
      weight  mass
      weight = constant · mass
   On the Earth’s surface, that constant, g, is
      9.8 Newtons/kilogram = 9.8 meters/second2
        (9.8 is approximately 10)
      32 feet/second2
      g is called the acceleration due to gravity
  Forces are measured in…
• Force = mass x acceleration, or F=ma
  (Newton’s 2nd law)
• Measured in Newtons (unit)
   • weight = mass x acceleration, and on
     earth, W=mg
• 1 Newton  1 kilogram·meter/second2
• A Newton is about ¼ pound, about the
  weight of a medium apple!
So, how does this effect me???
 Helps us in many ways
   Cars, rockets, merry-go-rounds
   Why objects fall down…
 Newton felt that the force of gravity was focused at the
 center of an object
   Intuition at first
   Needed to invent calculus to prove it
                   Center of mass
•Much of physics involves looking
for ways to simplify complicated
•An example is the motion of a
baseball bat thrown into the air.
•If one looks carefully, there is
a special point of the bat that
moves in a simple parabolic path.
•That special point is the center
of mass of the bat.

The center of mass of a body or a system of bodies
is the point that moves as though all of the mass were
concentrated there, and all external forces were
applied at that point.
Why things fall over….
  Every object has a special point called the center of
   gravity (CG). The CG is usually right smack in the center
   of the object.
  if the center of gravity is supported, the object will not
   fall over.
  Application !
      You generally want a running back with a low CG then it’s harder
       to knock him down.
  The lower the CG the more stable an object is.
 Going to use what we have learned about energy and
 forces to get us to the Moon and how roller coasters

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