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NEWTON’S LAWS OF MOTION

Aristotle’s Fallacy – Aristotle held the view that ‘an external force is required to keep a body in motion’.

Galileo’s Law of inertia – If the net force is zero, then a body at rest continues to remain at rest and a body in motion continues to move with uniform velocity. This property of the body is known as inertia. Galileo’s idea dethroned Aristotelian mechanics. Newton built on Galileo’s ideas and laid the foundation of mechanics in terms of three laws known as Newton’s laws of motion.

NEWTON’S FIRST LAW OF MOTION

“Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled by an external force to change that state”

Newton’ First law is also called law of Inertia as it defines inertia.

There are two parts to this statement – one which predicts the behavior of stationary objects and the other which predicts the behavior of moving objects. These two parts are summarized in the following diagram.

Illustrations of First law:

When a bus or a train starts suddenly, the passengers sitting inside tend to fall backwards. This is so because the lower part of his body starts moving with the bus or train but the upper part tries to remain at rest due to inertia of rest.
The dust particles in a carpet fall off when it is beaten with a stick. This is because the beating sets the carpet in motion whereas the dust particles tend to remain at rest and hence separate.
When a bus or train stops suddenly, a passenger sitting inside tends to fall forward. This is because the lower part of his body comes to rest with the bus or train but the upper part tends to continue its motion due to inertia of motion.
A person jumping out of a speeding train may fall forward. This is because his feet come to rest on touching ground and the remaining body continues to move due to inertia of motion.
When a stone tied to one end of a string is whirled and the string breaks suddenly, the stone files off along the tangent to the circle. This is because the pull in the string was forcing the stone to move in a circle. As soon as the string breaks, the pull vanishes. The stone in a bid to move along the straight line files off tangentially.

Question 1. Is it possible to have motion in the absence of force?

Ans. Motion requires no force and body can move with constant velocity even if the force acting on it is zero.

Question 2. A 10 kg object is moving horizontally with a speed of 2 m/s. How much net force is required to keep the object moving at this speed and in the same direction?

Ans. 0 N.

Momentum

Momentum is the measure of motion contained in a body. It is measured by the product of mass and velocity of the body.

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