Newton's laws by tangshuming

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									Newton’s Laws
Newton’s First Law of Motion

•   “Every object continues in its state of rest, or
    of uniform motion in a straight line, unless it is
    compelled to change that state of motion by
    forces impressed upon it ”

•   The tendency of matter to maintain its state
    of motion is known as INERTIA.
Newton’s First Law of Motion

•   Inertia prevents an object at rest from
    moving spontaneously or an object in
    uniform linear motion from changing its
    speed or direction spontaneously.

•   Force is simply a push or a pull.
Newton’s First Law of Motion

•   Changes in motion do not happen
    spontaneously.

•   Example:
    1)   Tablecloth under dishes
Newton’s First Law of Motion


Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
                   These pumpkins will not move unless acted on
motionless.        by an unbalanced force.
Forces

•   Usually think of a force as a push or pull
•   Vector quantity
•   May be a contact force or a field force
    •   Contact forces result from physical contact between two
        objects
    •   Field forces act between disconnected objects
         •   Also called “action at a distance”
Contact Forces…

•   Contact forces are forces which exert a
    push or pull (interact) while touching.
•   Examples:
    •   frictional forces,
    •   tensional forces,
    •   air resistance forces,
    •   applied forces (e.g., pushes or pulls).
Action at a Distance Forces…

•   Action-at-a-distance forces exert a push
    or pull (interact) without physical contact.
•   Examples:
    •   Gravity (e.g., when you jump off the ground,
        there is still a gravitational pull between you and
        the Earth),
    •   Electric (e.g., static electricity),
    •   Magnetic (e.g., two magnets).
Contact and Field Forces
    How Many FUNDAMENTAL Forces?


•   Four
     •   Strong Nuclear Force
     •   Electromagnetic Force
     •   Weak Nuclear Force
     •   Gravitational Force
What do they DO?

 •   Strong Nuclear Force
     •   Holds nucleus together
 •   Electromagnetic Force
     •   Interaction of electric & magnetic fields
 •   Weak Nuclear Force
     •   Causes radioactivity
 •   Gravitational Force
     •   Attracts mass
How Strong are they?

•   On a relative scale, we let the strong force
    = 1, then…

•   Strong Nuclear        1.00
•   Electromagnetic       1 x 10-2
•   Weak Nuclear          1 x 10-13

•   Gravitational         1 x 10-38
External and Internal Forces

•   External force
    •   Any force that results from the interaction
        between the object and its environment
•   Internal forces
    •   Forces that originate within the object itself
    •   They cannot change the object’s velocity
Example

          When the ball at the end of the
          string swings to its lowest point, the
          string is cut by a sharp razor.
          Which path will the ball then follow?
Example: Answer (b)

           When the ball at the end of the
           string swings to its lowest point, the
           string is cut by a sharp razor.
           Which path will the ball then follow?



 Answer:
 Inertia
Example


          When the pellet fired into
          the spiral tube emerges,
          which path will it follow?
          (Neglect gravity).
Example: Answer (b)

                  When the pellet fired into
                  the spiral tube emerges,
                  which path will it follow?
                  (Neglect gravity).



                  Answer: Inertia
 Newton’s Second Law of Motion

 •   A commonsense law.

                       F  ma
 •   The force on an object is equal to its mass multiplied by its
     acceleration.

 •   Mass is the amount of matter an object has.


 Acceleration is the time rate of change of velocity. (Velocity is speed
 •
   in a given direction.)
Newton’s Second Law of Motion

•   The more massive an object, the more inertia.

•   The more inertia an object has, the harder it is
    to get it moving.

•   Double the force - double the acceleration.
•   Double the mass - halve the acceleration.
Mass

•   …is the measure of the inertia of an object.
    •   Inertia is the tendency of an object to continue in its
        original motion
•   …is measured in kilograms.

•   … is how much matter is in an object..
    Weight


•   The weight of an object FW is the
    response of MASS to the
    GRAVITATIONAL FORCE.

•   FW = m g

•   g = 9.81 m/s2
    Units

•   The units of mass is the kilogram (in MKS
    system) and the slug (in Imperial system).
•   Mass is a SCALAR quantity.
•   The unit of force in the MKS system is the
    Newtons, where a force of 1 N will give a
    mass of 1 kg an acceleration of 1 m/s2 . In the
    Imperial system, the unit of force is the
    pound.
•   Force is a VECTOR quantity.
Units of Force

•   SI unit of force is a Newton (N)
                   kg m
             1N  1 2
                    s
•   US Customary unit of force is a pound (lb)
    •   1 N = 0.225 lb
Gravitational Force

•   Mutual force of attraction between any two
    objects
•   Expressed by Newton’s Law of Universal
    Gravitation:
                m1 m2
          Fg  G 2
                 r
•   G = universal gravitational constant =
    6.67x10-11 N-m2/kg2
Example

If two objects (m1 = 100 kg) and (m2 = 1000 kg) are
separated by 0.5m, what is the gravitational force
between them?
Example: Answer

         m1 m2
 Fg  G 2
          r
             11 N  m
                         2 
                            100kg1000kg
 Fg  
       6.67x10         2 
                  kg  0.5m    2


              5
 Fg  2.668x10 Newtons
Example: Answer

• Notice the very, very small answer.
  Gravity is the WEAKESS force by far.

• So - why are WE so aware of it?

• Because we are next to an incredibly
  large & massive object - the Earth.
Newton’s Second Law of Motion

•   Examples:
    1) Throwing a baseball
    2) Slingshot (David & Goliath)
    3) Hail
    4) Moving cars (1)
    5) Moving cars (2)
Example

If a mass of 1 kg is accelerated 1 m/s2 by a
force of 1 N, what would be the acceleration of
2 kg acted on by a force of 2 N?
      (a) 0 m/s2
      (b) 1 m/s2
      (c) 2 m/s2
      (d) 3 m/s2
      (e) unable to determine
Answer (b)

•   If a mass of 1 kg is accelerated 1 m/s2 by a
    force of 1 N, what would be the acceleration
    of 2 kg acted on by a force of 2 N?
                     F  ma
                     aF
                           m
                                   m
                     a  2N      1 2
                              2kg s
Newton’s Third Law of Motion

•   “To every action (force) there is always opposed an
    equal reaction (force): or, the mutual actions of two
    bodies upon each other are always equal, and
    directed to contrary parts. ”

•   The least intuitive law.

•   Every object that exerts a force on another object is
    always acted upon by a reaction force.
Newton’s Third Law of Motion

Examples:
  1.   Recoil of a gun.
  2.   Thrust of a rocket.
  3.   Rebound of a hammer from a struck nail.
  4.   Book sitting on a table.
  5.   Walking.
  6.   Lumberjack running on a log in a lake.
Example

 If a Mack truck
and a
Volkswagen
have a head-
on collision,
which vehicle      a) the Mack truck
will experience    b) the Volkswagen
the greater
impact force?      c) both the same
                   d) ... it depends on other factors
Example: Answer (c)

 If a Mack truck
and a
Volkswagen
have a head-
on collision,
which vehicle      Answer: Newton’s Third Law ~
will experience    action and reaction
the greater
impact force?
Example

           Consider the apple at rest
          on the table. If we call the
          gravitational force exerted
          on the apple action, what
          is the reaction force
          according to Newton's 3rd
          Law?
Example: Answer

                      Consider the apple at rest
                     on the table. If we call the
                     gravitational force (by the
                     earth) exerted on the
                     apple action, what is the
                     reaction force according
                     to Newton's 3rd Law?


Answer: The gravitational force exerted
by the apple on the earth
Example


If a mass of 10 kilograms is accelerated at
5 meters per second per second, what is
the force acting on it?
Example: Answer



             m    kg m
F  10 kg x 5  50 2 = 50 newtons
             s     s
               s
Example

What acceleration will result when a
12-N net force applied to a 3-kg
object?
    (a) 0 m/s2
    (b) 4 m/s2
    (c) 36 m/s2
    (d) Unable to determine
Example: Answer (b)

What acceleration will result when a
12-N net force applied to a 3-kg
object?       F  ma
              aF
                    m
                              m
              a  12N   3kg 4 2
                              s
Example

           A 1-kg rock is thrown at 20 m/s
          straight upward. Neglecting air
          resistance, what is the net force that
          acts on it when it is half way to the top
          of its path?
                  (a) 0 N
                  (b) 4.9 N
                  (c) 9.8 N
                  (d) 20 N
                  (e) Unable to determine
Answer (c)

             Answer: The only force
             acting on the rock is
             gravity.
                    F=ma
                    F = (1) (9.8)=9.8N
Classical Mechanics…



       The End

     (for now…)

								
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