Block ?A? in Fig weighs 2.40 N and block ?B? weighs 3.60 N. The coefficient of kinetic friction between all surfaces is 0.300. Find the magnitude of the horizontal force necessary to drag block ?B? to the left at constant speed (a) if ?A? rests on B? and moves with it (Fig. 1). (b) If ?A? is held at rest (Fig. 2). Figure 1: Figure 2:

Solution 73P Step 1 of 3: The block is moving with constant speed so the acceleration is zero(a=0) a)Total weight of the two blocks,w = w + A = 6 B F =ya y n=w=6 N f = n = 1.8 N k k F =xa x n=w=6 N F = f k 1.8 N Step 2 of 3: b)n and f normal and friction forces applied to the block B by the table top are the k same. fkB is the friction force that A applies to B.it is to the right because the force from A opposes the motion of B, n isBthe downward force that A exerts on B . fkA is the friction force that B applies to A.it is to the left because the block B wants A to move with it. n is the normal force that block B exerts on A. A By newton’s third law fkB = f kA and these forces are in opposite direction. Also, n = A thesB forces are in opposite directions. F =ya foryblock A gives n = w = A.4 N,Ao n = 2.4 N B f = n = (0.3)(2.4) = 0.72 N, and f = 0.72 N kA k A kB F =xa forxblock A gives T=f kA =0.72 N F =xa forxblock B gives F=f kB + fk =0.72 N +1.8 N =2. 52 N