21. If the block is sliding then we compute the kinetic friction from Eq. 6-2; if it is not sliding, then we determine the extent of static friction from applying Newton’s law, with zero acceleration, to the xaxis (which is parallel to the incline surface). The question of whether or not it is sliding is therefore crucial, and depends on the maximum static friction force, as calculated from Eq. 6-1. The forces are resolved in the incline plane coordinate system in Figure 6-5 in the textbook. The acceleration, if there is any, is along thexaxis, and we are taking uphill as +x. The net force along the yaxis, then, is certainly zero, which provides the following relationship: 0cosyNFFWθ=¡=¦GwhereW= mg= 45 N is the weight of the block, and = 15° is the incline angle. Thus, FN= 43.5 N, which implies that the maximum static friction force should be fs,max= (0.50) (43.5 N) = 21.7 N. (a) For ˆ( 5.0 N)iP=−G, Newton’s second law, applied to the
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This note was uploaded on 05/17/2011 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.