DYNAMICS

INTRODUCTION

In chapter 3, we discussed kinematics of moving bodies. In this chapter we will deal with dynamics of the bodies. Dynamics tells us answer to such question as: why do bodies move from rest and why do they accelerate or re accelerate? For such motion push or pull types external agency is required, which acts on a body in the motion either brings to the rest or external agency is required, which when acts on a body in a motion either brings it to  the rest or causes it to accelerate. On the other hand, if the external agency acts on a body at rest, the body will start to move. This pull or push is called force. Galileo Galilee (1564- 1642) studied the motion of bodies on an inclined plane and concluded that the body is continued to move with the same velocity if no external force acted on it. Later, Sir Isaac Newton (1642- 1727) studied the problem of motion in detail and enunciated three laws of motion, which cover the motion of all bodies- big or small. The three laws are named after him.

1.   NEWTON’S LAWS OF MOTION

The following three laws of motion were originally formulated by Newton and were first published in 1686.

1.   First law of motion: Every body continuous in its rest or of uniform motion in a straight line, unless external force act on it.

2.   SECOND LAW OF MOTION: The rate of momentum of a body is directly proportional to the impressed force and takes place in the direction in which the force acts.

3.   THIRD LAW OF MOTION: Action and reaction are always equal and opposite to each other.

2.   INTERPRETATION OF THE FIRST LAW

The first law of motion has two parts:

I)                Every body continues in its state of rest, unless external force acts on it.

II)            Every body continues in its state of uniform motion in a straight line, unless external force acts on it.

The first parts of the law are experienced everyday, because a body lying on a table continues to remain there until it is pushed (or pulled).

The second part of the law may seem contrary to our common experiences. A rolling ball comes to rest after sometimes. It should be realized that the ball comes to rest because of air- resistance and also due to the friction from the surface it rolls on.