91Ó°ÊÓ

Acceleration is a vector quantity as it specifies both direction and magnitude. It is defined as the rate of change of velocity with respect to time.

The motion of a rock from the cliff is free fall. If the air resistance is neglected, the rock moves under the effect of gravity and accelerates with a constant acceleration of $9.8\mathrm{m}/{\mathrm{s}}^2$.

When the elevator makes stops while moving from the second to the fifth floor, it first accelerates from rest and reaches a certain speed. To stop at the fifth floor, it decelerates. Therefore, the acceleration of the elevator is not constant. Its magnitude and direction change during the starts and stops.

According to Newton’s first law, an object at rest has zero velocity. A dish kept on a table remains in its state of motion, i.e., at rest. The dish has zero velocity and hence constant acceleration (zero).