A ball of mass M is on a horizontal surface with negligible friction. One end of a string is attached to the ball so that it is spun in a horizontal circle of radius R at a tangential speed vo, as shown in Figure 1. A free-body diagram for the ball at an instant in time is shown in Figure 2. The magnitude of the tension force is also indicated in Figure 2. Which of the following equations represents the centripetal acceleration of the ball if its tangential speed is increased to 2vo ?