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Newton’s 3 rd Law and Momentum

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Newton’s 3 rd Law When one object exerts a force on a second object, the second object exerts a force on the first that is equal in strength and opposite in direction.

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Action and Reaction Another way of stating Newton’s 3 rd Law is “to every action force there is an equal and opposite reaction force”. Example – You exert a downward force on a trampoline. The trampoline exerts an equal force upward on you.

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Action and Reaction Forces Don’t Cancel! Action and reaction forces work on different objects, so they are not balanced. Example – A swimmer pushes the water. She “acts” on the water. The water “reacts” by moving and pushing her forward.

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Momentum Momentum – how much force is needed to change an object’s motion. Depends on an object’s mass and velocity Momentum is given the symbol p.

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Momentum Equation p = mv p = momentum in kg * m/s m = mass in kg v = velocity in m/s Just like velocity, momentum has size and direction!

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Example Momentum Problem At the end of a race, a sprinter with a mass of 80 kg has a speed of 10 m/s. What is the sprinter’s momentum?

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Force and Changing Momentum If you catch a baseball, your hand might sting (even with a glove). This is because the baseball exerted a force on your hand when it came to a stop and its momentum changed. This can also be calculated!

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Calculating the Force Using Momentum F = (mv f – mv i ) / t F = force in N mv f = final momentum in kg * m/s mv i = intital momentum in kg* m/s t = time

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Example of Calculating Force Using Momentum What is the force exerted by a catcher’s glove on a 0.15 kg baseball moving at 35 m/s that is stopped in 0.02 s?

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Law of Conservation of Momentum The momentum of an object does not change unless it’s mass, velocity, or both change. Momentum can be transferred from one object to another. Example – A bowling ball traveling down the lane has momentum. That momentum is transferred to the pins as the ball strikes the pins.

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