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Action and Reaction Notes

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					Action and Reaction
       Notes
    Newton’s Third Law of
        Motion
          Action and Reaction
 Newton’s  Third Law of Motion: when one
 object exerts a force on a second object, the
 second one exerts a force on the first that is
 equal in size and opposite in direction. OR…
 for every action force there is an equal and
 opposite reaction force.

 These  forces always act in pairs.
 These forces always act on DIFFERENT
  objects.
          Newton’s Third Law
Examples:
- jumping on a trampoline
 (you place a force on the trampoline which places
 an equal and opposite force back on you – causing you to go “up”)


- jumping from a boat
 (you place a force on the boat which sends the boat
 backward, but the boat places an equal and opposite
 force back on you causing you to move forward)

- a rocket
 (gases are pushed out by the engine which place
 an equal and opposite force back on the rocket –
 causing it to fly)
Action Force/ Reaction Force Diagram




Reaction Force




                         Action Force
                Momentum
 Momentum:   a property a moving object has
 because of its mass and velocity

    Momentum = mass x velocity
       p=mxv

 Momentum   is measured in kg x m/s
 Example:
 - two trucks may have the same velocity, but
 the one that has the greater mass will have
 greater momentum.
       EXAMPLES of Momentum
 Ifthe velocity of a bus & a car traveling
  along the highway is equal, the bus will
  have more momentum because it has
  more mass.

                  MORE          LESS
 If the mass of two bicycles and their riders
  are equal, the one that is moving at a
  greater velocity has more momentum
  because it is moving faster.
    Law of Conservation of Momentum
 Law of Conservation of Momentum: the total amount
  of momentum of a group of objects does not change
  unless outside forces act on the objects.
 Example: Pool table
      - no balls moving = zero momentum

      - cue ball moving toward the group of balls has
    momentum

        - cue ball hits the group of balls and scatters the
    balls; spreads the total momentum out amongst all of
    the moving pool balls
    Law of Conservation of Energy
        applied to momentum
 Energy cannot be created or destroyed, but can
  change forms.
 Momentum is a result of an object’s motion.
 Energy can be transferred from one moving
  object to another in a collision.
 All energy within the system can be accounted
  for because any energy left after the transfer of
  energy from one object to another in a collision
  changes to different forms of energy.
                       EXAMPLES
                   In a game of billiards or pool:
   Potential energy for the motion of balls on a pool table is
    stored in the person holding the stick as he/she prepares
    to hit the ball.
   Most of this energy is changed to kinetic energy (of the
    moving cue ball).
   When the cue ball collides with the group of balls
    causing them to scatter and spread out, kinetic energy of
    the cue ball is shared among all moving balls.
   Remaining energy is changed to sound, heat from
    friction, etc.
   In conclusion, all potential energy originally stored in the
    person holding the pool stick is used.
   Therefore, no energy is created or destroyed.

				
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