CBSE Class 9 Answered

Force can be defined as the external agent, which changes or tends to change the state of rest or motion of an object. It has a push or pull effect.

Force can change the speed of a moving object:  A goalkeeper applies force to stop a football and reduce its speed to zero. If he is not able to stop the ball properly, it may only slow down and not come to a position of rest. When a player kicks a moving football, it starts moving faster. A goalkeeper also slows down or stops a football by applying force in the opposite direction to which football is moving. On the other hand, to move the ball faster, the player applies a force in the same direction in which the ball is moving. Thus a force can change the speed of a moving body.

Force can change the direction of a moving object: The football player can change the direction of a football moving towards him by kicking it so that it moves towards the goalpost. Thus a force can change the direction of a moving body.