Velocity

Velocity is defined as the rate of change of displacement with respect to time.

Since displacement is a vector quantity, velocity is also a vector quantity.

Therefore, even when the direction of a body at constant speed changes, the body is not said to be in a constant velocity.

Hence, when estimating the number of times velocity changes, we must keep in mind the magnitude as well as the direction.

When therefore is a change in velocity, the body undergoes an acceleration which means that an external force has been applied.

Change in Velocity

When the football is kicked by the first player, the velocity of the football changes from zero to a certain value. Here, the player imparts a force on the football through his kick.

When the football is kicked by the second player in the direction of the goal post, the direction of the ball is changed, and hence the velocity changes. Here, the player imparts a force on the football through his kick.

When the ball is collected by the goalkeeper, the football comes to rest and hence the velocity becomes zero. Here, the ball imparts force on the goalkeeper while the goalkeeper collects the ball with his hand.

When the football is kicked by the goalkeeper, the velocity of the football changes from zero to a certain value. Here, the goalkeeper imparts a force on the football through his kick.

Hence, the number of times the velocity of the football changes is 4.