Newton's First Law of Motion & "Inertia"

 

Before Galileo, scientists were under the presumption that a force is needed not only to start a motion but also to keep an object moving even with uniform velocity. In other words, an object remains in motion so long as the external force applied to produce motion remains present.

Galileo, however, did not approve of the above view. He stated that an object once in motion will stay in motion even when the force applied to set the object in motion is withdrawn, provided that there is no other force such as friction present to oppose the motion. For example, when a ball is kicked, it moves a certain distance and then comes to rest. It stops due to the friction between the ground and the ball. If the friction between the ground and ball is reduced, the ball would have been able to move a comparatively longer distance before coming to rest. If the friction between the ground and ball could be reduced to zero, the ball would have remained in motion forever after kicking it once.

Thus, it is concluded that in the absence of friction, no force is required to keep an object moving after bringing it in motion. An object, once set in motion, moves with uniform velocity if no force acts on it. This is called Galileo's law of inertia.

• A body at rest stays in the state of rest until an external force is applied on it.
• A body in motion continues moving in the same direction with the same speed unless a force is applied on it.

Newton put the above observations in the form of a law, called Newton's First law of motion.

According to Newton's first law of motion, if a body is in the state of rest, it will remain in the state of rest, and if it is in the state of motion, it will continue to move with the same speed in the same direction unless an external force acts upon it.

INERTIA

Inertia is the inherent property of an object by virtue of which it has the tendency to resist the change in its state of rest or state of motion. The greater the mass, the greater is the inertia of the body. Therefore, mass is a measure of inertia.

There are two kinds of inertia:

I) Inertia of rest: If a body is at rest, it will remain in the state of rest unless an external force is applied on it to change its state of rest.

• When a train suddenly starts moving forward, the passenger standing in the compartment tends to fall backwards. This is because the lower part of the passenger's body is in close contact with the train. As the train starts to move, the passenger's lower part shares the motion at once, but the upper part due to inertia of rest cannot share the motion simultaneously and so it tends to remain in the same place. The lower part moves forward first, leaving the upper part behind. Hence, the passenger falls backwards.
• When a hanging carpet is beaten with a stick, the carpet moves along with the stick, but the dust particles remain in their state of rest due to inertia and then they fall down due to the earth's gravitational pull.
• When a tablecloth that has objects placed on it is pulled off the table sharply, the objects remain on the table, and the cloth is removed. This is due to the inertia of rest of the objects on the table, due to which they remain in their position and land on the table due to gravity after the cloth is pulled away.

II) Inertia of motion: A body in a state of motion continues to be in the state of motion with the same speed in the same direction in a straight line unless and external force is applied on it to change its state.

• When brakes are abruptly applied on a fast-moving cycle, it leans or topples forward since the front of the cycle comes to rest at first, but the back is still in motion due to inertia.
• When a running car stops suddenly, the driver tends to lean forward as the lower part of him comes to rest with the car at once, but the upper part was still in motion. 
• When a passenger jumps out of a moving train, he falls down. Due to inertia, the passenger was still moving along with the train and as soon as his feet touch the platform, the lower part of him comes to rest while the upper part was still in motion. To avoid falling, the passenger should start running for a certain distance in the direction of motion as soon as his feet touch the ground.

In conclusion:

• Inertia is the property of an object by virtue of which it tends to retain its state of rest or of motion.
• Newton's first law of motion states that a body in the state of rest will remain in the state of rest, and if it is in the state of motion, it will continue to move with the same speed in the same direction unless an external force acts upon it.