Newton’s First Law of Motion or Law of Inertia

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Newton’s first law of motion states that every body continues in its state of rest or of uniform motion in a straight line, unless it is compelled by some external force to change that state.

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Explanation of Newton’s First Law of Motion:

Newton’s first law consists of three parts:

(i) Without applying an external force, a body at rest continues to be at rest. This part is self-evident. For example, we find in day-to-day life that every object continues to lie where it is unless we move it.

(ii) Without applying an external force, a body in uniform motion continues to move uniformly. This part is slightly difficult to realize because in everyday life, 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 traveling some distance. In fact, motion of bodies is being opposed by the invisible forces like air resistance and the force of friction between the body and the ground. If these forces were removed, a body moving uniformly shall never stop on its own.

(iii) Without the application of an external force, a body cannot change its direction of motion, i.e., it continues to move along the same straight line. 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 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 of uniform motion along a straight line, unless it is acted upon by an external force to change its state. This means force applied on a body alone, can change its state of rest or state of uniform motion along a straight line.

Hence, we define force as an external effort in the form of a push or pull that moves or tries to move a body at rest; stops or tries to stop a body in motion; changes or tries to change the direction of motion of a body.

Hence, Newton’s first law defines force.

Newton’s First Law Defines Inertia

Since, Newton’s first law of motion states that, a body continues to be in a state of rest or of uniform motion along a straight line, unless it is acted upon by an external force to change its state.

This means that a body, on its own, cannot change its state of rest or state of uniform motion along a straight line. This inability of a body to change by itself its state of rest or state of uniform motion along a straight line is called inertia of the body. Hence Newton’s first law defines inertia and is rightly called the law of inertia.

Quantitatively, inertia of a body is measured by the mass of the body. Heavier the body, greater is the force required to change its state and hence greater is its inertia. The reverse is also true.

Types of Inertia

Inertia of a body is of three types:

Inertia of rest.
Inertia of motion.
Inertia of direction.

1. Inertia of rest

It is the inability of a body to change by itself, its state of rest. This means a body at rest remains at rest and cannot start moving on its own. For example:

When a bus or train starts suddenly, the passenger sitting inside tends to fall backward. 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.
When a horse starts suddenly, the rider tends to fall backward on account of inertia of rest of the upper part of the body as explained above.
The dust particles in a durree fall off when it is beaten with a stick. This is because the beating sets the durree in motion whereas the dust particles tend to remain at rest and hence separate.
Place a coin on a smooth piece of cardboard covering a glass. Strike the cardboard piece suddenly with a finger. The cardboard slips away and the coin falls into the glass. This happens on account of inertia of rest of the coin.
When we shake a branch of a mango tree, the mangoes fall down. This is because the branch comes in motion and the mangoes tend to remain at rest. Hence they get detached.

2. Inertia of motion

It is the inability of a body to change by itself its state of uniform motion i.e., a body in uniform motion can neither accelerate nor retard on its own and come to rest. For example:

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.
When a horse at full gallop stops suddenly, the rider falls forward on account of inertia of motion as explained above.
A person jumping out of a speeding train may fall forward. This is because his feet come to rest on touching the ground and the remaining body continues to move due to inertia of motion.
An athlete runs a certain distance before taking a long jump. This is because velocity acquired by running is added to the velocity of the athlete at the time of the jump. Hence he can jump over a longer distance.

3. Inertia of direction

It is the inability of a body to change by itself its direction of motion i.e., a body continues to move along the same straight line unless compelled by some external force to change it. For example:

An umbrella protects us from rain. It is based on the property of inertia of direction. The raindrops falling vertically downwards cannot change their direction of motion and wet us, with the umbrella on.
When a stone tied to one end of a string is whirled and the string breaks suddenly, the stone flies 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 is unable to move along the straight line and flies off tangentially.
When a car rounds a curve suddenly, the person sitting inside is thrown outwards. This is because the person tries to maintain his direction of motion due to directional inertia while the car turns.
The rotating wheels of any vehicle throw out mud, if any, tangentially, due to directional inertia.

Applications of Law of Inertia

The following examples from our daily life illustrate the law of inertia:

When a running horse suddenly stops, the rider falls forwards.
Because the lower part of the body of the rider (which is in contact with the horse) comes to rest; but because of inertia, the upper part of the body tends to keep on moving. As a result of it, the rider falls forwards.
When a horse suddenly starts moving, the rider falls backwards.
Because the lower part of the body of the rider (which is in contact with the horse) comes in motion; but because of inertia, the upper part tends to be at rest. Hence, the rider falls backwards.
We hit a carpet with a stick to remove the dust.
On hitting with a stick, the carpet comes in motion; but because of inertia, the dust particles remain at rest. Due to this, dust particles get removed from the carpet.
If a cloth placed under a book is given a sudden pull, it goes out without disturbing the book.
Because the book continues to be at rest due to inertia, when the cloth is suddenly pulled out.
Athletes run some distance before taking a long jump.
When an athlete runs some distance, the velocity acquired due to inertia is added to the velocity of the athlete at the time of jump. Since the length of the jump depends on the initial velocity, the athlete is likely to jump a long distance by doing so.
A man jumping from a moving train may fall down.
Because as his feet touch the ground, they come to rest; but due to inertia, the remaining part of the body tends to keep on moving. As a result, he may fall down in the direction of the motion of the train.
A ball thrown upward in a train moving with uniform velocity, returns to the thrower.
Because during the upward and the downward journey; due to inertia, the ball also moves along horizontally with the velocity of the train. Hence, it covers the same horizontal distance as the train does, and the ball returns to the thrower.
When we shake the branch of a mango tree, the mangoes fall down.
Because due to shaking, the branches come into motion; but due to inertia, the mangoes continue to remain at rest and get detached.
The sparks coming out of a grinding stone are tangential to the rotating stone.
It is due to directional inertia. Once the sparks are produced, no external force acts on the sparks, and in the absence of external force, their direction of motion cannot change. The sparks so produced follow paths tangential to the grinding stone.
The mud from the wheels of a moving vehicle flies off tangentially.
It also happens for the reasons explained above. In order that the flying mud does not spoil the clothes of passersby, the wheels are provided with mudguards.