practice+2b+solutions

# practice+2b+solutions - Physics 140 Fall 2010 Second...

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Physics 140, Fall 2010, Midterm 2 Problem 1: A variable force (a force in the y-direction whose magnitude depends on the position) is acting on an object that is moving along the following path: (1) it starts at the origin and moves along the x-axis to a point x = 2m, y = 0; (2) it moves parallel to the y- axis to a point x = 2m, y = 3m; (3) it moves parallel to the x-axis to a point x = 0, y = 3m; (4) it moves parallel to the y-axis back to the origin. What is the total work done for the complete round trip? A. -12J B. 0 C. 12J D. 18J E. 27J Along A 0 0 0 = = = A W F y Along B 27 3 6 ˆ 6 2 3 0 2 3 0 = = = = = y ydy W j y F x A G J Along C 0 lar to pererndicu is = C W l d F G G Along D 0 0 0 = = = D W F x 27 = total W J. Answer E. Problem 2: Two bull frogs of mass 1kg and 2kg, respectively, sit at either end of a uniform narrow rectangular plate of mass 3kg and length 10m. The plate is at rest on the surface of a calm lake and has negligible friction with the water. The two frogs manage to trade ends without falling in the water. How far (in meters) does the plate move on the surface of the water? A. 0 B. 0.67 m C. 1.34 m D. 1.67 m E. 2.01 m Before After 1 kg 1 kg 2 kg 2 kg 3 kg 3 kg 5 m 10 m (0,0) (0,2) (2,3) (0,3) A B C D 3 F xyj = ± ±
From the left side of the plate before: 6 35 2 3 1 ) 10 ( 2 ) 5 ( 3 ) 0 ( 1 = + + + + = kg kg kg x CM m From the left side of the plate after: 6 25 2 3 1 ) 10 ( 1 ) 5 ( 3

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practice+2b+solutions - Physics 140 Fall 2010 Second...

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