91Ó°ÊÓ

Velocity is a vector quantity and is defined as the time rate of change of displacement of an object. On the other hand, acceleration is the time rate of change of velocity.

Suppose an object is moving in the forward direction. A force starts acting on it in the opposite direction of its motion. It decreases the velocity of the object and continues to do so until it approaches zero.

While observing the object, you will see that it starts accelerating in the reverse direction due to the force. At this point, the velocity becomes zero. And at any time before and after this point, the velocity is non-zero.

The change in velocity continues even when the object acquires zero velocity. Thus, acceleration occurs simultaneously.

For example, a ball is projected upward from the surface of the Earth with a constant velocity. The force of gravitation acts opposite (downward) to the motion of the ball. At the highest point, the velocity of the ball becomes zero, and the ball starts falling with an acceleration of $9.8\mathrm{m}/{\mathrm{s}}^2$.