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Published byJonathan Cooper Modified over 9 years ago
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Momentum the quantity of motion § 8.1–8.2
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What’s the point? Nature keeps careful account of momentum.
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Think Question Which process will require more time? A.Pulling as hard as you can to accelerate a little red wagon from rest to a speed of 1 m/s. B.Pulling as hard as you can to accelerate a horse trailer from rest to a speed of 1 m/s. C.The two take the same amount of time.
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Think Question Which process will require more force? A.Accelerating a little red wagon from rest to a speed of 1 m/s in ten seconds B.Accelerating a horse trailer from rest to a speed of 1 m/s in ten seconds. C.The two take the same force.
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Impulse where t is duration F Is net force Impulse is a vector. Impulse = F t Effect of a force over time
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Think Question Two objects of different masses move at the same velocity. Which one is harder to stop? A.The light object. B.The massive object.
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Think Question Two objects with the same mass move different speeds. Which one is harder to stop? A.The fast object. B.The slow object.
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Momentum Formula momentum is a vector. p = mv
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Whiteboard Work Prove that when a net force F is applied to an object of mass m for a time t, its change in momentum is p = F t
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Impulse–Momentum Theorem Impulse = F t. F t= m v= ma t So F t = (mv) = change in momentum
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Newton’s second law As Newton had it: dt dp F =
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Poll Question Which changes its momentum the most? A.A moving object that stops when it hits a barrier. B.A moving object that bounces back from a barrier. Momentum is a vector. Which receives the greater impulse? Hints:
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Rebound and Momentum initial pfinal p final p – initial p initial p final p = 0 final p – initial p pp pp
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Poll Question Which changes its kinetic energy the most? A.A moving object that stops when it hits a barrier. B.A moving object that bounces back from a barrier. Hints:Kinetic energy is a scalar. Which has the greater net work done?
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Conservation of Momentum Here today, somewhere tomorrow § 8.2
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Poll Question If a 0.25-g insect collides with a 1250-kg compact car, which experiences the greatest impulse in the collision? A.The insect. B.The car. C.It’s a tie. D.Insufficient information to answer. Hint: Remember Newton’s third law.
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Poll Question If a 0.25-g insect collides with a 1250-kg compact car, which experiences the greatest momentum change in the collision? A.The insect. B.The car. C.It’s a tie. D.Insufficient information to answer. Hint: Remember the impulse-momentum theorem.
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Conservation of Momentum Newton’s first law: –no outside force –no change in v –thus no change in p So an isolated object’s momentum never changes.
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Conservation of Momentum Newton’s third law: –interacting objects apply equal and opposite impulses to each other –they experience equal and opposite momentum changes So the sum of their momenta remains the same. ∑p i = ∑p f
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Conservation of Momentum The total momentum of an isolated system never changes.
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