101.F07.lecture9.multi

# 101.F07.lecture9.multi - PHY101 The power of conservation...

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PHY101: The power of conservation laws: Momentum! http://johnfenzel.typepad.com/john - fenzels - blog/images/2007/03/29/safety - net - cartoon.jp What is momentum? Start with Newton’s third law: ~ F 21 = - ~ F 12 . Multiplying Newton’s third law by Δ t and using Newton’s second law gives us ~ F 21 Δ t = - ~ F 12 Δ t = m 2 Δ ~ v 2 = - m 1 Δ ~ v 1 . If we define momentum ~ p = m ~ v , then we have Δ ~ p 2 = - Δ ~ p 1 . Note that momentum is a vector. Momentum problem A 2.0 kg block is moving to the right at 1.0 m/s just before it strikes and sticks to a 1.0 kg block initially at rest. What is the total momemtum of the two blocks after the collision? Momentum problem A 2.0 kg block is moving to the right at 1.0 m/s just before it strikes and sticks to a 1.0 kg block initially at rest. What is the total momemtum of the two blocks after the collision? Answer: 2.0 kg m/s to the right. Impulse-momentum theorem The product of ~ F Δ t is known as impulse. Assuming the forces acting on an object are constant, Δ ~ p = X ~ F Δ t . This equation is a statement of the impulse-momentum theorem. Big force-Short time versus Small force-Long time concept.

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