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Unformatted text preview: 1 Chap.6 HW1 Questions 1. In three experiments, three different horizontal forces are applied to the same block lying on the same countertop. The force magnitudes are F 1 = 12 N, F 2 = 8 N, and F 3 = 4 N. In each experiment, the block remains stationary in spite of the applied force. Rank the forces according to (a) the magnitude f s of the static frictional force on the block from the countertop and (b) the maximum value f s,max of that force, greatest first. a) F net = ma Since the block is stationary v = constant and a = 0. Therefore F- f s = 0 F = f s . and the ranking is F 1 , F 2 , and F 3 b) f s,max = s F N = s mg in this problem and does not depend on the horizontal force. Therefore they all tie. 2. In Figure 6-14, horizontal force 1 F v of magnitude 10 N is applied to a box on a floor, but the box does not slide. Then, as the magnitude of vertical force 2 F v is increased from zero, do the following quantities increase, decrease, or stay the same: (a) the magnitude of the frictional force s f v on the box; (b) the magnitude of the normal force N F v on the box from the floor; (c) the maximum value f s,max of the magnitude of the static frictional force on the box? (d) Does the box eventually slide? Hint: Draw a free-body diagram of the mass. There will be a total of four forces. Now apply Newtons 2 nd law. Note that in a), b), and c) the box has constant velocity (actually v = 0)so a = 0. a) Applying Newtons 2 nd law along x: F 1 f s = 0 F 1 = f s F 2 does not affect f S and so f S stays the same. b) Applying Newtons 2 nd law along y: F N F 2 mg = 0 F N = F 2 + mg Thus as F 2 increases F N also increases. c) Since f s,max = s F N , f s,max increases d) Since f s,max increases and f S is constant, the box will not move. 6. In Figure 6-16, a block of mass m is held stationary on a ramp by the frictional force on it from the ramp. A force F v , directed up the ramp, is then applied to the block and gradually increased in magnitude from zero. During the increase, what happens to the direction and magnitude of the frictional force on the block? ...
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