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Momentum

Momentum. Chapter 12.3 Notes. Newton’s Third Law of Motion. Newton’s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

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Momentum

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  1. Momentum Chapter 12.3 Notes

  2. Newton’sThirdLaw of Motion • Newton’s Third Law of Motion • When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. • The two forces are called action and reaction forces. They are equal in size and opposite in direction.

  3. Action and Reaction Forces • Action and Reaction Forces • Action and reaction forces always come in pairs. Whenever there is an action there must be an equal and opposite reaction.

  4. Ex. 1) Two bumper cars colliding Ex. 1) Two bumper cars colliding • Action Force: Car 1 hitting and exerting a force on car 2. • Reaction Force: Car 2 exerting an equal and opposite force on car 1.

  5. Ex. 2) Pressing your hand against the wall Ex. 2) Pressing your hand against the wall • Action Force: Your hand exerting a force on the wall. • Reaction Force: The wall exerting an equal and opposite force against your hand.

  6. Ex. 3) Driving a nail into wood Ex. 3) Driving a nail into wood • Action Force: The nail exerting a force on the wood. • Reaction Force: The wood exerting an equal and opposite force on the nail.

  7. Action-Reaction Forces and Motion • Action-Reaction Forces and Motion • Some action-reaction forces will cause motion. Ex. 4) Swimming • Action Force: Swimmers pushing against water with their hands. This causes the water to move in the direction of the swimmers hand. • Reaction Force: The water exerting an equal and opposite force on the swimmers hand. This causes the swimmers to move forward through the water.

  8. Action – Reaction Forces • Can action-reaction forces cancel to a net force of zero? • NO!!! The action and reaction forces do not act on the same object. In ex. 4 the action force acted on the water and the reaction force acted on the swimmer. • Only when forces act on the same object can the net force be equal to zero.

  9. Momentum • Momentum • Momentum is the product of an object’s mass and its velocity. • An object has a large momentum if the product of its mass and velocity is large. However, when an object is at rest its momentum is equal to zero.

  10. Example 5 Ex. 5) A large boulder and a small marble • If a large boulder and a small marble are slowly rolling toward you which would be easier to stop? • The marble!!! • The marble is easier to stop because it has a much smaller mass.

  11. Example 5 • Now assume the marble is rolling one hundred times faster than the boulder. Which would be harder to stop? • It depends… Which object has a larger product f its mass x velocity. The larger the product the larger the momentum. The larger the momentum the harder it is to stop the object.

  12. Momentum • The formula for momentum • Momentum = Mass x Velocity; where mass is measured in kg and velocity is measured in meters per second. This means the unit for momentum is kg-meters per second.

  13. Example 6 Which has more momentum? A 0.046 kg golf ball with a velocity of 60.0 m/s or a 7.0 kg bowling ball with a velocity of 6.0 m/s.

  14. Example 7 Which has more momentum? A 75 kg runner moving at 2.5 m/s or a 95 kg runner moving at 2.0 m/s?

  15. Example 8 Which has a larger velocity? A 2,000 kg car with a momentum of 50,000 kg-m/s or a 16,000 kg semi with a momentum of 30,000kg-m/s.

  16. Example 9 A class studied the speed and momentum of a 0.25 kg ball dropped from a bridge. The graph shows the momentum of the ball from the time it was dropped until the time it hit the river flowing below the bridge. * At what time did the ball have zero momentum? * At what time did the ball have the greatest momentum? * What is the ball’s speed after 1.25 seconds? (2.5, 6.5) 7 6 5 4 3 2 1 1.5 2 0 .5 1 2.5

  17. Conservation of Momentum • The Law of Conservation of Momentum • States that in a closed system, the momentum does not increase nor decrease. • In a closed system, if no net force acts on a system, then the total momentum of the system does not change.

  18. Conservation of Momentum • In a closed system, the loss of momentum of one object equals the gain in momentum of another object. Therefore the momentum is conserved. • A closed system means other objects and forces cannot enter or leave a system.

  19. Example 10: Train Wreck • Before Collision After Collision

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