What is Force
2.1. INTRODUCTION
A In the last chapter, we studied motion of a body along a straight line in terms of its velocity, acceleration and distance travelled by the body in a particular time. We know that motion can be uniform or non-uniform. In the present chapter, we shall try to understand the causes of motion. Do all motions require a cause ? If so, what is the nature of this cause ? Why does the speed of an object change (increase or decrease) with time ? How can the direction of motion of a body be changed ? And so on.
We observe that when a ball on ground is given a small hit, the ball does not move for ever. It does come to rest after some time. This observation suggests that the state of rest is the natural state of an object. And state of motion is not the natural state of the object. This belief persisted till Galileo Galilei and Sir Isaac Newton developed their theories to understand all about motion.
2.2. WHAT IS FORCE?
In our day-to-day life, we find that an effort is required to move a body at rest or to stop a moving body. Often, this effort is in the form of a muscular effort applied on the body to change its state of rest or motion. The concept of force is based on this. In fact, no one has seen force or tasted force. However, we see or feel the effect of a force. Therefore, concept of force can be explained by describing what happens when some force is applied on an object. Some of the common effects of force are :
1. A force may move a body at rest. For example, when we kick a football kept on the ground with our foot, the football moves. The force applied by our foot moves the ball. Again, when we push a wall, it does not move. The force applied by us has only tried to move the wall, but it has not succeeded.
2. A force may stop a moving body. For example, when a player catches a moving cricket ball, the force applied by player's hands stops the moving ball. Again, when a bullet pierces through a target, the target exerts force on the bullet, but fails to stop it.
3. A force can change the speed of a body. For example, when a ball is dropped from a certain height, the speed of the ball goes on increasing due to gravitational pull of the Earth on the ball, in the downward direction. Thus force applied by the Earth in the direction of motion of the ball increases the speed of the ball. On the contrary, when a ball is thrown vertically upwards, gravitational pull of the Earth on the ball goes on decreasing the speed of the ball. Thus force applied by the Earth in a direction opposite to the direction of motion of the ball, decreases the speed of the ball.
4. A force may change the direction of a moving body. For example, when a moving cricket ball is hit by a bat, the cricket ball moves in a different direction. The force applied by the bat changes the direction of the moving cricket ball. Similarly, when a moving tennis ball is hit by a racket, it goes in a different direction. The force applied by the racket changes the direction of the moving tennis ball. Again, when we blow air from our mouth on the smoke from a burning incense stick (agarbatti), the direction of motion of smoke changes due to force exerted by the blowing air. 5. A force may change the size and shape of a body. For example, when we press a rubber ball or a balloon between our two hands, the shape of the rubber ball or balloon changes from spherical to oblong as shown in Fig. 2.1. The forces applied by our hands change the shape of the rubber ball/balloon.
Again, Fig. 2.2(a) shows the original shape and size of a light spring. When we stretch the spring by applying outward forces on the two ends of the spring, the length of the spring increases and its shape also changes, as shown in Fig. 2.2(b). When the spring is compressed by applying inward forces on the two ends of the spring, the length of the spring decreases and its shape also changes, as shown in Fig. 2.2(c).
 At home, we find that the size and shape of dough (kneaded flour) changes on pressing it with a rolling pin(belan) while making chapatis. The shape of a tooth paste tube and an ointment tube changes when we squeeze the tube by applying force on it.
Similarly, the shape and size of kneaded wet clay (gilli mitti) change when a potter makes different pots out of it.
From the foregoing examples, we conclude that :
Force is an external effort in the form of pushing, pulling, stretching, compressing etc., which may move a body at rest or stop a moving body or change the speed of a body or change the direction of a moving body or change the size and shape of a body.
Important point which must be learn |
1. In the last chapter, we studied motion of a body along a straight line. In this chapter, we shall understand the causes of motion.
2. The cause of motion is the force. No one has seen force or tasted force. However, we see or feel the effect of a force. Hence, concept of force is explained by describing what happens when some force is applied on an object.
3. Force is an external effort in the form of pushing, pulling, stretching, compressing etc., which may move a body at rest or stop a moving body or change the speed of a body or change the direction of a moving body or change the size and shape of a body.
4. The forces are of two types : Balanced forces and Unbalanced forces,
5. If the resultant of all the forces acting on a body is zero, the forces are called balanced forces.
The balanced forces cannot change the state of rest or state of uniform motion or the direction of motion of the body. They can however, change the shape and size of the body. 6. If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces.
The unbalanced forces can change the state of rest or state of uniform motion or the direction of motion of the body or size and shape of the body.
7. An unbalanced force is needed to move a body from state of rest. But no such force is required to maintain uniform motion of the body. An unbalanced force is also required for changing speed of the body or for changing the direction of motion of the body. |
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