EFFECT OF FORCE ON STATE OF MOTION

EFFECT OF FORCE ON STATE OF MOTION

Let us now find out what happens when a force acts on an object.

Effect of force on an object

1. Force moves the object at rest
2. It increases or decreases the speed of an object
3. It also changes the direction of motion of an object
4. It stops a moving object

Activity 11.3(a): ( ball move-rest )

*   Take a rubber ball and place it on a level surface such as a tabletop or a concrete floor.

*   Now, gently push the ball along the level surface (Fig. 11.6).

*   Does the ball begin to move? Push the ball again while it is still moving. Is there any change in its speed? Does it increase or decrease?

*   Next, place your palm in front of the moving ball. Remove your palm as soon as the moving ball touches it.

*   Does your palm apply a force on the ball? What happens to the speed of the ball now? Does it increase or decrease?

*   What would happen if you let your palm hold the moving ball?

Figure 11.6: A ball at rest begins to move when a force is applied on it

You might recall similar situations. For example, while taking a penalty kick in football , the player applies a force on the ball. Before being hit, the ball was at rest and so its speed was zero. The applied force makes the ball move towards the goal. Suppose, the goalkeeper dives or jumps up to save the goal. By his action, the goalkeeper tries to apply a force on the moving ball. The force applied by him can stop or deflect the ball, saving a goal being scored. If the goalkeeper succeeds in stopping the ball, its speed decreases to zero.

These observations suggest that a force applied on an object may change its speed. If the force applied on the object is in the direction of its motion, the speed of the object increases. If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object.

Activity 11.3(b): ( tyre increases speed )

You have seen children competing with one another in moving a rubber tyre or a ring by pushing it (Fig. 11.7). You now understand why the speed of the tyre increases whenever it is pushed.

Figure 11.7: To move a tyre faster it has to be pushed repeatedly

Activity 11.4: ( force-direction-ball )

Paheli is curious to know whether the application of a force can only change the speed of an object. Let us find out.

*   Take a ball and place it on a level surface as you did in Activity 11.3.

*   Make the ball move by giving it a push [Fig. 11.8(a)].

*   Now place a ruler in its path as shown in Fig. 11.8(b).

*   In doing so, you would apply a force on the moving ball.

*   Does the ball continue to move in the same direction after it strikes the ruler?

*   Repeat the activity and try to obstruct the moving ball by placing the ruler in such a way that it makes different angles to its path.

*   In each case note your observations about the direction of motion of the ball after it strikes the ruler.

Figure 11.8: (a) A ball set in motion by pushing it along a level surface and (b) the direction of motion of the ball after it strikes the ruler placed in its path

Let us consider some more examples. In a game of volleyball, players often push the moving ball to their teammates to make a winning move. Sometimes the ball is returned to the other side of the court by pushing or smashing it. In cricket, a batsman plays his or her shot by applying a force on the ball with the bat. Is there any change in the direction of motion of the ball in these cases? In all these examples the speed and the direction of the moving ball change due to the application of a force. Can you give a few more examples of this kind?

A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion. Thus, a force may bring a change in the state of motion of an object.

Does it mean that the application of a force would always result in a change in the state of motion of the object? Let us find out.

It is common to experience that many a time application of force does not result in a change in the state of motion. For example, a heavy box may not move at all even if you apply the maximum force that you can exert. Again, no effect of force is observed when you try to push a wall.

State of Motion

The state of motion of an object is described by its speed and direction of motion. The state of rest is considered to be the state of zero speed. An object may be at rest or in motion; both are its states of motion

Source: This topic is taken from NCERT TEXTBOOK