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05quiz_11 - Physics 9A-C Quiz#5 Name Last 4 ID ....

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Physics 9A-C Quiz #5 Name: 4/29/2011 Last 4 ID: A small block with mass 0.120 kg is attached to a cord passing through a hole in a frictionless, horizontal surface as shown. The block is originally revolving at a distance of 0.40 m from the hole with a speed of 0.70 m/s. The cord is then pulled from below, shortening the radius of the circle in which the block revolves to 0.10 m. At this new distance, the speed of the block is observed to be 2.80 m/s. a) (3 pts) What is the tension in the cord in the initial situation when the block has speed = 0.70 m/s ? Summing the horizontal forces on the block, there is only the tension which provides the centripetal acceleration: ÍF = F T = m v 1 2 r a) F T = m v 1 2 r = 0.12 kg ( ) 0.7 m/s ( ) 2 0.4 m = 0.15N b) (3 pts) What is the tension in the cord in the final situation when the block has speed v 2 = 2.80 m/s? At the later radius and speed, the tension is F T = 0.12 kg 2.80 m/s ( ) 2 0.1 m = 9.41 N c) (4 pts) How much work was done by the person who pulled the cord? The surface is frictionless and horizontal, so the net work is the work done by the string.
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