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mt2sol_11b - Phyiscs 9A-C White Midterm 2 Solutions...

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Phy 9A-C, Midterm 2 Solutions, Page 1 of 2 5/20/11 White f F T F N mg Phyiscs 9A-C Midterm 2 Solutions Cole White [1] a) Í F √

 = 0 b) always, c) both, d) position, e) equal to, f) spin slower, g) p m < p M , h) Watt, i) 0, j) all of the above. [2] a) (10 pts) Use the work energy theorem W T = ÎK. W F T = F T Δ x = 500 N ( ) 5 m ( ) = 2500 J While F N does no work it is needed for the friction force: Í F y = F N - mg cos 36.9° = 0 õ F N = mg cos36.9 ° = 4 5 mg õ W f = μ k F N Δ x = μ k 4 5 mg Δ x õ W f = 0.35 ( ) 4 5 30 kg ( ) 9.8 m/s 2 ( ) 5 m ( ) = 411.6 J . The only other force doing work is gravity: W g = mg h = 30 kg ( ) 9.8 m/s 2 ( ) 3 m ( ) = 882.0 J where h = (5 m)sin36.9° õ W T = 2500 J - 411.6 J + 882 J = 2970 J. Work energy õ W T = ÎK = 1 2 mv 2 õ v = 2 2970 J ( ) 30 kg = 14.1 m/s b) (5 pts) The center of mass is at 0.5 m from the bottom. The lever arm for the tension force is r = 0.8 m - 0.5 m = (0.3 m)(500 N) = 150 N·m c) (5 pts) The lever arm for the tension force points in the same direction as the force; hence, r √
 ª F √
 = 0 õ = 0.
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