Questions from inside the chapter

Class 9
Chapter 9 Class 9 - Force and Laws Of Motion

## In the following example, try to identify the number of times the velocity of the ball changes:

“A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the

opposite team collects the football and kicks it towards a player of his own team”.

Also identify the agent supplying the force in each case.

Velocity of ball can change if

Speed of the ball changes

Direction of the ball changes

Now, lets see the different cases the velocity of the ball changes

Case 1

“football player kicks a football to another player of his team”

In this case, the first football player changes the direction of the ball as he kicks it.

Hence, velocity changes.

The force is applied by the first football player.

Case 2

“another player of his team who kicks the football towards the goal”

In this case, the second football player changes the direction of the ball as he kicks it towards the goal.

Hence, velocity changes.

The force is applied by the second football player.

Case 3

“The goalkeeper of the opposite team collects the football”

Goalkeeper collects the football means the goalkeeper stops the football.

Since the speed of the ball changes , hence velocity changes.

The force is applied by the goalkeeper in a direction opposite to that of motion.

Case 4

“and kicks it towards a player of his own team”

In this case, the goalkeeper changes the speed of the ball from zero to some speed.

Since the speed of the ball changes , hence velocity changes.

The force is applied by the goalkeeper.

Hence, in the given example, the velocity of the ball changes four times.