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BELLWORK 11/12/2015

BELLWORK 11/12/2015. A car has a mass of 1.50 x 10 3 kg. If the force acting on the car is 6.75 x 10 3 N to the east, what is the car ’ s acceleration?. ANSWER. 4.50 m/s 2 , east. The Third Law: Action and Reaction Pairs. “ For every action there is an equal and opposite reaction. ”

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BELLWORK 11/12/2015

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  1. BELLWORK 11/12/2015 • A car has a mass of 1.50 x 103kg. If the force acting on the car is 6.75 x 103N to the east, what is the car’s acceleration?

  2. ANSWER • 4.50 m/s2, east

  3. The Third Law: Action and Reaction Pairs “For every action there is an equal and opposite reaction.” This statement is known as Newton’s third law of motion. Newton’s third law discusses Action and Reaction Pairs.

  4. Forces occur in pairs The two forces in a pair are called action and reaction. Anytime you have one, you also have the other. If you know the strength of one you also know the strength of the other since both forces are always equal.

  5. Name the Action – Reaction Force Pairs • A baseball pushes a glove left.

  6. Example #2 • A bowling ball pushes a pin to the left

  7. Example #3 • The exhaust gases from the rocket push down against the ground.

  8. Example #4 • Identify the two pairs of action-reaction forces. • Use the notation "foot A", "foot C", and "ball B" in your statements.

  9. ANSWER • The first pair of action-reaction force pairs is: foot A pushes ball B to the right; and ball B pushes foot A to the left. The second pair of action-reaction force pairs is: foot C pushes ball B to the left; and ball B pushes foot C to the right.

  10. Newton's Third Law Newton’s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction. Action and reaction forces act on different objects, not on the same object. Action and Reaction forces do not cancel each other out because they act on different objects.

  11. EXPLAIN • Name the action and reaction forces present when a hammer hits a nail. • If the force that the nail exerts on the hammer is equal to the force that the hammer exerts on the nail, why doesn’t the nail remain at rest?

  12. Now • Read Page 133 – Action and Reaction forces each act on different objects.

  13. Hmmmmmm……….. Astronauts go on spacewalks when they need to work on their space shuttle or perform experiments. Once they exit the space shuttle, how can they begin their motion if there is nothing for them to push on?

  14. ANSWER • One solution is to throw something opposite the direction you want to move.

  15. Try this Problem…….. • While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

  16. ANSWER • They are the same…For every action, there is an equal and opposite reaction.

  17. CLASSWORK • Read Pages 132- 134 • Answer #’s 1-3 on Page 134.

  18. BELLWORK 11/13/15 • A boy pushes a lawnmower to the right • A. Name the reaction force. • B. Draw 2 FBD’s to show how the action and reaction forces cancel out AND why both the boy and the lawnmower don’t just stay still.

  19. ANSWER ACTION FORCE REACTION FORCE F ground pushing foot forward Fpush from boy F friction F lawnmower on boy

  20. HMMM……….. • Action and Reaction Pairs do not cancel each other out because each force acts on a different object. They would both have to act on the same object to cancel each other out. • Horse and Wagon Problem

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