This is a concept question from a previous final physics exam. The correct answer is (b) and my professor confirms this, but I can't understand why the answer is not (d) since the question seems to imply that the ball is already in rolling motion and that friction would not play a part. A ball is rolling freely on a flat surface. (a) The force of friction points opposite to the motion of the centre of mass. (b) The force of friction points in the direction of motion of the centre of mass. (c) The force of friction cannot be determined. (d) The force of friction is zero. (e) None of the above. You throw a tennis ball of radius R on the floor such that it rolls freely on a flat surface for some time and then it hits a patch of ice where the coefficient of friction between the ball and the ice is s = 0. When the ball hits the patch (a) It continues to roll as before. (b) It starts to spin (omega > Vcm/R) (c) It begins to skid (omega < Vcm/R) (d) none of the above The correct answer for this one is (c) but I can't understand why its not (a). This is likely due to the same error I am making in the first question.