[SOLVED] Please clarify questions on forces and energy Hi everybody, I have been doing some reading and practice problems in my AP book lately, so I would appreciate it if people could tell me if the statements below are correct. 1. A force applied perpendicular to the velocity can only cause a change in direction, not in speed. The force also does no work. 2. The centripetal acceleration formula, ac = v^2/r, applies even when speed changes. 3. When a person tosses a ball held by a string in a vertical circle, the speed constantly changes, so the magnitude of the centripetal force and the tension in the string differs at various points as well. 4. A pendulum is also a case of centripetal acceleration even if it does not go in a complete circle. The centripetal force throughout is equal to T - m*g*cos(theta). m*g*sin(theta) causes the tangential acceleration.