9. The force of friction acts upon a baseball player as he slides into third base.

1. We first note that friction opposes the motion of the player. When the player slides into third base, his velocity is directed along his motion, and the force of friction acts in the opposite direction. 2. The kinetic friction force is given by \[ f_k = \mu_k N \] where \(\mu_k\) is the coefficient of kinetic friction and \(N\) is the normal force. For a player on a horizontal field, the normal force is \[ N = mg, \] with \(m\) being the mass of the player and \(g\) the acceleration due to gravity. 3. Because friction acts opposite to the displacement, the work \(W\) done by friction over a displacement \(d\) is \[ W = f_k d \cos(180^\circ) = -f_k d. \] Substituting for \(f_k\) we obtain \[ W = -\mu_k mgd. \] The negative value indicates that friction removes energy from the system (i.e., it does negative work). 4. According to the work-energy theorem, the net work done on the player is equal to the change in his kinetic energy: \[ \Delta KE = W_{\text{net}}. \] Since the work done by friction is negative, it reduces the player’s kinetic energy, slowing him down as he slides. Thus, the key takeaway is that the force of friction does negative work, reducing the kinetic energy of the sliding baseball player as he comes to a stop at third base.