Which on the following objects has the least kinetic energy at the bottom of the incline if they have the same mass and radius: A) cylinder B) sphere C) hoop D) all have the same I piced B but the answer is D. My question is, the sphere has the least rotational kinetic energy, which means it will roll slowly, making it's kinetic energy less than the hoop or cylinder. I dont understand how they can all have the same kinetic energy when the hoop has a greater rotaional kinetic energy. When a ball is fired from a cannon wth a mass much greater than that of the ball, which of the following is true? The answer is "The center of mass of the system remains unchanged." I picked "The ball and the cannon have equal momentum after the ball is fired" I picked this because if the cannon and ball represents an elastic collision. I dont see why my answer isnt valid.