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# Assign3 - PHY 53 Summer 2010 Assignment 3 Reading Dynamics...

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Assignment 3 Reading : Dynamics 1 Key concepts : Reference frames, inertia, mass, force, Newton’s laws of motion, friction. 1. A skilled oarsman rows his boat back and forth across a river of width w , and then the same distance down and upstream. The water flows at rate V , and the oarsman can row at speed v in still water. a. Find the time t D it takes him to row downstream and back again. Ans: 2 w v 1 1 ( V / v ) 2 . b. To row directly across the river he must aim his boat upstream at an angle α relative to the line he wants to follow. What is α in terms of given quantities? Ans : sin α = V / v . c. Find the time t C it takes him to row directly across the river and back. Ans: 2 w v 1 1 ( V / v ) 2 . d. The oarsman knows v , and measures the two times. Find the speed V of the river’s flow in terms of v and the ratio r = t C / t D . Ans : v 1 r 2 2. The strings in this problem will break if the tension is too large. a. The blocks are being accelerated along a frictionless horizontal surface by pulling on string A. As the acceleration is increased, which string breaks first? Explain. b. Now a block is suspended by a string from a ceiling as shown. As the tension in string A is gradually increased, which string breaks first? Explain.

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