WHAT ARE NEWTON'S LAWS OF MOTION and Galileo's Observation ?

 

GALILEO'S OBSERVATIONS   Galileo arrived at some important conclusion about motion of objects by observing their motion on an inclined plane.   Galileo observed that when a small marble ball rolls down an ideal, frictionless plane inclined on both sides, [Fig. 2.4),   (i) velocity of the ball goes on increasing due to unbalanced force of helping gravity. On reaching the bottom of the incline after covering a distance s in falling through a height h, the ball has acquired a definite velocity   (ü) the ball then climbs up the inclined plane on the right. The velocity of the ball goes on decreasing due to unbalanced force of opposing gravity.   (iii) when the inclination 0, of right side plane is same as the inclination of left side plane, distance covered by the ball before stopping is the same, i.e., s, = s if o, = 0.   (iv) when angle of inclination of right side plane is reduced to 02, distance covered by the ball before stopping increases, i.e. 82 > S1, when 2 <= ).   (v) when angle of inclination of right side plane is reduced further to 03, the distance covered by the ball before stopping increases further, i.e., sz > $2>s, when 0; <<, (= 0). = Note that in all the three cases, height attained by the ball before stopping is exactly the same (= h) through which the ball had descended over the left side plane. 32 9 3   (vi) If right side plane were made horizontal, the marble ball would continue to travel for ever, trying to reach the same height from which it was released. This is as if the unbalanced force on the marble ball is zero.   From these observations, Galileo Concluded that an unbalanced external force is required to initiate the motion (from state of rest), but no unbalanced net force is needed to sustain the uniform motion, i.e., these observations, Galileo concluded that an unbalanced external force required to initiate objects move with a constant speed along a straight line when no force acts on them.   In actual practice, it is difficult to achieve the state of zero unbalanced force, because of the presence of frictional forces which oppose the motion. Therefore, in the presence of frictional forces, some external force is required to move objects with constant speed along a straight line.

NEWTON'S LAWS OF MOTION

 

Newton investigated further the ideas put forth by Galileo regarding motion of three fundamental laws that govern the motion of objects. These three laws are known as Newton's Laws of Motion. We shall discuss these laws of motion one by one, and learn some of their important applications.

 

NEWTON'S FIRST LAW OF MOTION

 

According to Newton's first law of motion, a body continues to be in a state of rest or in a state of uniform motion along a straight line, unless some external unbalanced force is applied on the body to change that state.

 

The law consists of three parts :

 

(i) A body at rest continues to remain at rest until some external unbalanced force is applied on the body to move it . This is what we find in daily life. A book lying on a table continues to lie there only till someone removes it. A table, chair, sofa set, bed etc. continue to lie where they are unless we apply a force to move them.

 

(ii) A body in uniform motion continues to move uniformly unless an external force is applied to change its speed. This part of the law is difficult to realise as we find that a ball rolling on the ground does stop after some time. Similarly, when the engine of a moving car is switched off, it stops after travelling some distance . In fact, motion of everybody is being opposed by the invisible forces like air resistance and friction between the body and the ground. If these opposing forces were removed, a body in uniform motion will continue to move uniformly, and never stop on its own.

 

(iii) A body moving along a straight line will continue to move along the same straight line unless an external force is applied on the body to change its direction of motion. For example, to turn a car moving over a straight road, we have to apply force on the steering wheel of the car. Similarly, a bike cannot change its straight line path on its own. We have to turn its handle.

 

NEWTON'S FIRST LAW DEFINES FORCE

 

According to Newton's first law of motion, a body continues to be in a state of rest or in a state of uniform motion along a straight line, unless an external force is applied on the body to change the state. This means that force applied on a body alone can change its state of rest or state of uniform motion along a straight line. Hence, we may define force as an external effort in the form of a push or pull which

 

(i) actually moves or tries to move a body at rest,

 

(ii) actually stops or tries to stop a moving body,

 

(iii) actually changes or tries to change the direction of motion of the body.

 

This is how Newton's first law defines force.

 

INERTIA AND MASS

 

According to Newton's first law of motion, a body continues to be in a state of rest or in a state of uniform motion along a straight line, unless an external force is applied on the body to change its state.

 

The law means that a body, on its own, cannot change its state of rest or state of uniform motion along a straight line. In other words, all objects resist a change in their state or the objects oppose the forces that try to change their state. This property is called Inertia. Hence,

 

Inertia of a body is the inability of the body to change by itself its state of rest or state! uniform motion along a straight line. Thus, Newton's first law of motion defines Inertia and is rightly called the Law of Inertia.