In a laboratory experiment on friction, a 135-N block resting on a rough horizontal table is pulled by a horizontal wire. The pull gradually increases until the block begins to move and continues to increase thereafter. ?Figure E5.26 shows a graph of the friction force on this block as a function of the pull. (a) Identify the regions of the graph where static friction and kinetic friction occur. (b) Find the coefficients of static friction and kinetic friction between the block and the table. (c) Why does the graph slant upward at first but then level out? (d) What would the graph look like if a 135-N brick were placed on the block, and what would the coefficients of friction be?

ANSWER: The weight of the block is, N = mg = 135 N . Where N is the normal reaction and also defined as weight. a) In the graph at the point (75,75) static friction occurs and at the point (75,50) kinetic friction takes place. b) We know that, the static friction is, F max= S N= S = S 135. As the graph shows that, when the frictional force reaches 75 N, the pull remains constant and the frictional force decreases too. 75 N = ×S135 =S75/135 = 0.555 The kinetic friction also obeys same type of equation, which is, F = K N = KN = K 135 According to the graph,...