Newtons First Law Of Motion states that everybody continues in its state of rest or of uniform motion in a straight line unless compelled by some external force to change its initial state.
Newtons first law of motion may be split up into two parts-
If a body is at rest, it continues to remain in its state of rest unless it is acted upon by some external force. This fact is quite understandable in light of our daily experience. The table, chair, book, radio and other material bodies continue lying at the same place till they are disturbed by some external agency.
If a body is in motion, it remains moving with uniform velocity in a straight line unless some external agency acts on it. Though it appears that a moving ball stops after some time without applying any external force, the frictional force of the earth’s surface and the resistance of the air are actually responsible for it.
The property by virtue of which a body remains in a state of rest or of uniform motion is known as inertia. Thus, inertia is the ability of a body to change by itself its state of rest or state of uniform motion in a straight line. Newton’s first law of motion is also called the law of inertia.
Inertia is of different forms-
Inertia of Rest:
It is the tendency of a material body to maintain its state of rest. For illustration, consider following examples-
When a passenger is sitting in a bus and the bus starts moving forward suddenly, the passenger experiences a jerk backwards. This is due to the reason that when the bus starts moving suddenly, the lower part of the body in contact with the bus is set into motion in the forward direction but the upper part continues to remain in the state of rest because of inertia of rest.
When a woollen coat is beaten with a stick, dust particles inside it fall off. It is due to the reason that the coat is set into motion all of a sudden while the dust particles tend to continue to be at rest.
Inertia of Motion:
It is the tendency of a material body to maintain its state of uniform motion. For illustration, consider following examples-
When the running boy stumbles, his body has a tendency to fall in front because of the reason that his feet come to rest suddenly while the body is still in a state of motion.
A man sitting in a running bus experiences a jerk in the forward direction when the bus stops suddenly.
When a man jumps out of moving train, he falls with his head forward. This is due to the fact that on jumping out of moving train, his feet come to rest suddenly while the rest of his body tends to remain in motion.
Inertia of Direction:
It is the tendency of a material body in motion to maintain its direction. For illustration, consider following examples-
If a stone fastened at one end of a piece of string is whirled and the string breaks, the stone flies off along the tangent of the circle described by the swirling string.
The sparks from a grinding stone are always tangential to the stone in motion.
Newtons First Law of Motion and the concept of Force:
Newtons first law of motion gives us the qualitative definition of force.
According to Newtons first law of motion, the change in the state of rest or of uniform motion can be caused only due to some external factor called force. Thus, force may be defined as an influence which causes changes or tends to change the state of rest or the state of uniform motion of a body in a straight line. It is the force due to which the train is pulled by the engine, the iron filings are attracted by the magnet, the stone is displaced from its original position, the ball is rolled over the ground, and so on.