A ball of mass m moving with velocity v_i_vec strikes a vertical wall. (Figure 1) The angle between the ball's initial velocity vector and the wall is theta_i as shown on the diagram, which depicts the situation as seen from above. The duration of the collision between the ball and the wall is Deltat, and this collision is completely elastic. Friction is negligible, so the ball does not start spinning. In this idealized collision, the force exerted on the ball by the wall is parallel to the x axis. What is the final angle theta_f that the ball's velocity vector makes with the negative y axis?