Practice Problem 4 A student is running down the hall in his socks at 10 m/s when he stops running and slides to a stop. If it takes him 2 s to stop, what is the coefficient of friction (u) between his socks and the oor? B. 0.5 Givens: v1=10mls v2=0 At=2s Unknown: a = ? E ation' vzvl qu ' '1- At . _ _ 0 lOm/s _ 2 2 Substitute and Solve. a 2S5 m/s or 5 n1/s [backward] Here, the net force acting to stop the student is the frictional force: m a=Ff Also, since there is no vertical applied force, the normal force is equal to the weight, so: F f=M F N = u mg Set these two expressions for the frictional force equal to each other and solve for u: m a=um g > u= (Mass cancels out!)