sy12_oct15_07hc - Physics 207 Lecture 12 Physics 207...

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Physics 207 – Lecture 12 Physics 207: Lecture 12, Pg 1 Physics 207, Physics 207, Lecture 12, Oct. 15 Lecture 12, Oct. 15 Agenda: Finish Chapter 9, start Chapter 10 Agenda: Finish Chapter 9, start Assignment: c HW5 due Wednesday HW5 due Wednesday c HW6 posted soon HW6 posted soon x Collisions x Momentum conservation in 2D x Impulse Physics 207: Lecture 12, Pg 2 c Transition from forces to conservation laws Transition from forces to conservation laws Newton’s Laws b Conservation Laws Conservation Laws b Newton’s Laws They are different faces of the same physics phenomenon for special “cases” Physics 207: Lecture 12, Pg 3 Lecture 12, Exercise 1 Momentum Conservation Momentum Conservation A. Box 1 B. Box 2 C. Same c Two balls of equal mass are thrown horizontally with the same initial velocity. They hit identical stationary boxes resting on a frictionless horizontal surface. c The ball hitting box 1 bounces elastically back, while the ball hitting box 2 sticks. x In which case does the box ends up moving fastest ? x No external force then, vectorially, COM 1 2 Physics 207: Lecture 12, Pg 4 Lecture 12, Exercise 1 Momentum Conservation c Which box ends up moving fastest ? c Examine the change in the momentum of the ball. In the case of box 1 the balls momentum changes sign and so its net change is largest. Since momentum is conserved the box must have the largest velocity to compensate. (A) (A) Box 1 (B) (B) Box 2 (C) (C) same 1 2 Physics 207: Lecture 12, Pg 5 A perfectly inelastic collision in 2 -D c Consider a collision in 2-D (cars crashing at a slippery intersection. ..no friction). v 1 v 2 V before after m 1 m 2 m 1 + m 2 c If no external force momentum is conserved. c Momentum is a vector so p x , p c Elastic means that the objects do not stick. c There are many more possible outcomes but, if no external force, then momentum will always be conserved c Start with a 1-D problem. Before After
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sy12_oct15_07hc - Physics 207 Lecture 12 Physics 207...

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