The velocity of football changes four times.
First, when a football player kicks to another player, second when that player kicks the football to the goalkeeper. Third when the goalkeeper stops the football. Fourth when the goalkeeper kicks the football towards a player of his own team.
The velocity of the ball changes in the following manner
(i) When the first player kicks the football, its speed changes from zero to a certain value. As the ball was at rest initially the velocity was zero at that stage.
(ii) When another player kicks the ball towards the goal post, the direction of the ball gets changed. As the direction changes the velocity of the ball varies. Therefore, its velocity also changes.
(iii) When the goalkeeper collects the ball, the speed of the ball reduces to zero from a certain value. The ball comes to rest hence the velocity comes down to zero. So again, the velocity of the ball has changed.
(iv) The goalkeeper kicks the ball towards his team players. Hence, the speed of the ball increases from zero to a certain value. Hence, its velocity increases to some value and thus changes once again.
Agent supplying the force:
a) First case is First player
b) Second case is Second player
c) Third case is Goalkeeper
d) Fourth case is Goalkeeper