One end of a string is tied to the ceiling of an elevator, and the other end is tied to...

One end of a string is tied to the ceiling of an elevator, and the other end is tied to a rock (Fig. Q3.20). The elevator is moving in such a way that the tension in the string is zero. (a) What is the acceleration of the rock? (b) Explain why you cannot find the velocity of the rock, even though you can find its acceleration.