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Newton ’ s 3 rd Law of Motion. Newton ’ s 3 rd Law of Motion. For every action, there is an equal and opposite reaction In other words, when object A exerts a force on object B, object B exerts an equal and opposite force on object A. Example:
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Newton’s 3rd Law of Motion • For every action, there is an equal and opposite reaction • In other words, when object A exerts a force on object B, object B exerts an equal and opposite force on object A. Example: • Walking - You push against the floor, the floor pushes on you. • Swimming – You push against the water, the water pushes on you.
Action and Reaction Forces • Interchangeable • I act and you react or you act and I react. • Simultaneous • No lag time • Equal/Opposite • Same magnitude, opposite direction • Act on different objects
Identifying Action and Reaction Forces • First identify the interacting objects • Then decide how they interact • You punch a wall • You high-five your friend • A cue ball hits the 8-ball • A boulder falls to earth
Action and Reaction on Different Masses In the case of the falling boulder, does the earth also fall to the boulder? Yes! The Earth has MUCH more mass than the boulder so the Earth’s movement is undetectable.
Why don’t action/reaction forces cancel? Consider a horse and a cart.
MAC Truck and VW bug • Same impact force but different affect because of mass. • What about a bug on a windshield?
1st Law Force Pairs – act on the same object • The Earth pulls down on a box • The ground pushes up on the box • 3rd Law Force Pairs – act on different objects • You push on a box • The box pushes on you
Liftoff! Rockets don’t have wings, how do they propel themselves into space? U SA Action: The rocket pushes on the air Reaction: The air pushes on the rocket