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Assign7 - PHY 53 Summer 2010 Assignment 7 Reading Systems...

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Reading : Systems of Particles Key concepts : Center of mass, momentum, motion relative to CM, collisions. 1. Two identical balls of mass m are propelled at t = 0 from the same starting point. Ball A moves on a frictionless horizontal surface, starting with speed v 0 . Ball B is thrown into the air with initial speed 2 v 0 , at elevation angle 60°. a. What is the initial velocity (magnitude and direction) of the CM? Ans : v CM (0) = ( 7 /2) v 0 ; tan 1 ( 3 /2) 41 ° above horizontal. b. What is the total external force on the system? Ans : m g . c. What is the acceleration (magnitude and direction) of the CM? Ans : 1 2 g . d. Sketch on the drawing the trajectories of the balls and of the CM, while ball B is in Fight. e. Indicate on your sketch the location of the CM at the instant when ball B has its maximum height. 2. Some questions about frictionless ice. a. A pole is standing on end on frictionless ice as shown. Its CM is at its midpoint. A slight nudge makes it slip and fall. On the drawing sketch its con±guration when it is lying on the ice, and explain your choice. b. Batman and Robin have been stranded by one of the arch-±ends in the middle of a lake frozen with frictionless ice. Give a method by which they can reach the shore. c. A child is standing at the front of a large raft resting on frictionless ice. She walks to the back, then stops. Does the ±nal location of the raft

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Assign7 - PHY 53 Summer 2010 Assignment 7 Reading Systems...

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