The discovery of the laws of motion was a dramatic moment in the history of science. Before Newton’s time, the motions of things like the planets were a mystery, but after Newton there was complete understanding.

The contribution of Newton was three laws:

Þ      The first law describes what happens when the body is left alone or is not disturbed.

Þ      The second law gives a specific way of determining how the velocity changes under different influences called forces.

Þ      The third law characterizes the basic nature of force.

OBJECTIVE

In this chapter we will study the three important laws of motion given by Newton. The study includes the dynamics of particle under influence of different forces.

After studying this chapter we will be able to answer the question like Why can’t we drive fast on icy road, in cricket how hard should a player hit the ball for a sixer or how does a small engine pull a large train and many other similar questions.

PREQUISITE

We know by experience that all bodies in nature interact in some way with one another.

Before actually going into details of the three Laws of motion we need to get familiar with  “force”. It was believed and has been accepted that “force” is an external or internal agent present to “influence” the natural state of motion of an object. So this is an influence (force) needed to change the natural state of body; that is of rest or of uniform motion.

Having this idea of force we can now readily move to the Newton’s first law of motion.

Newton’s first law of motion

It states that every object persists in its natural state of motion i.e. continues to be at rest or moves in a straight line with uniform (constant) velocity. (This is what is meant by natural state of motion); In the absence of a net external force acting (impressed) on it.

Mathematically, it is equivalent to say that for producing acceleration (that is for changing velocity) in a body, we need to have a net external force. (By net external force we mean vector sum of all the external forces acting on it)

It can be easily deduced from the statement of change in the state of motion. It is directly related to a frame of reference about which we have discussed earlier. To mark the point here, we can discover that by viewing objects from different frame of references the natural state of motion as perceived by different observers will be obviously different (can only be same if the frames are truly equivalent). Therefore, the change in state will also depend on the choice of reference frame. Finally, the amount of acceleration produced in a body (or change in velocity) will depend on our choice of reference frames.

A mass ‘M’ is lying (figure 3.1) on a table which is at rest (w.r.t. table). Explain its state with the help of Newton’s First Law of motion.

Since ‘M’ is lying on a table, there is no external force acting on it (forget about gravity just for the immediate discussion). As per Newton’s first law of motion it will keep on lying at rest with respect to table for infinite time.

Here, comes out a very important, intrinsic (that is inherent) property of a body which is that it retains its state of motionlessness (as well as of motion, if its in motion) which is termed as INERTIA of an object. This is present in all materialistic bodies in this universe.