Newton’s First Law of Motion or Law of Inertia Physics Notes

Newton’s First Law of Motion or Law of Inertia:
Sir Isaac Newton’s first law of motion describes the behaviour of a massive body at rest or in uniform linear motion i. e., not accelerating or rotating.

The first law states, “A body at rest will remain at rest and a body in motion will remain in motion unless in the abesence of external force.” It is also called the law of inertia.

This simply means that things can not start, stop or change direction all by themselves. It requires some force acting on them from the outside to cause such a change. While this concept seems simple and obvious to us today, in Newton’s time it was truely revolutionary.

NCERT Solutions Guru Newton’s First Law of Motion or Law of Inertia Physics Notes

Following inferences can be easily drawn from Newton’s first law of motion:

  • If a body is at rest, then so as to set it in motion an external force has to be applied on it.
  • If a body is moving with a constant speed along a straight line. Then in order to increase or decrease its speed, an external force has to be applied in the direction of motion or opposite to the direction of motion.
  • If a body is moving with a constant speed along a straight line, then in order to change its direction of motion an external force has to be applied in a direction normal to the direction of motion.
    From the above discussion, it follows that the first law of motion is simply the law of inertia.

Application of Law of Inertia:
1. A ball thrown upward in a train moving with a uniform velocity, returns to the thrower:
Because during the upward and the downward journey, due to inertia, the ball also moves along the horizontal with the velocity of the train. Hence, it covers the same horizontal distances as the train does and the ball returns to the thrower.

2. When a running car suddenly stops, the rider falls forwards: Because the lower part of the body of the rider (which is in contact with the car) comes to rest. But due to of inertia, the upper part of the body tends to keep moving. As a result of it the rider falls in the forward direction.

3. When a car suddenly starts moving, the rider falls in backward: Because the lower part of the body of the rider (which is constant with the car) comes in motion, but due to the inertia the upper part tends to be at rest. Hence, the rider falls in backward.

4. Dust is removed from a having carpet by beating it with a stick: The carpet comes into motion, but because of inertia the dust particles remain at rest, due to this, the dust particles get removed from the carpet.

5. An athlete runs for a certain 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 to jump. Since length of the jump depends upon the initial velocity, athlete is likely to jump a longer distance by doing so.

Physics Notes