Newton’s laws of motion

Sir Isaac Newton (1642-1727):

  • Sir Isaac Newton, an English scientist, enunciated the basic laws of motion.
  • He was born at Woolsthrope, Lincolnshire, England on the Christmas day of 1642.
  • He extended the concept of force from terrestrial to celestial phenomena.
  • Newton discovered the Universal law of Gravitation.
  • He also studied light including its decomposition into various colors; the phenomenon of Newton’s rings.
  1. Newton’s first law of motion or law of inertia:
  • According to Newton’s first law or law of inertia, a body remains in state of rest or uniform motion in a straight line until an external force is applied on it.
  • To change from state of rest to state of motion and vice-versa, external force is required.
  • Inertia is the property by virtue of which bodies resist the change of state of rest or motion.
  • The measurement of inertia is i.e. heavier mass offers more resistance compared to smaller one.

Few examples:

  • If we are seated inside a bus at rest and bus suddenly starts, we tend to fall back. This is because; our lower body part comes to state of motion immediately as it is in contact with the bus, but upper part of our body was still in the state of rest and opposes the change and hence we tend to fall. This is called inertia of rest.


  • When a moving bus comes to halt, we tend to fall forward. This is because; when bus comes to rest, our lower body which is in contact with the bus also stops, whereas, the upper body is still in the state of motion and keeps moving and hence we fall forward. This is called inertia of motion.
  1. Newton’s second law of motion:
  • According to Newton’s second law of motion, rate of change of momentum of a body is directly proportional to the force applied in the direction of force.

F α (mv-mu)/t [mv= final momentum, mu= initial momentum, t=time taken]

F= k.m(v-u)/t

F= [ since (v-u)/t =a, i.e rate of change of velocity {(v-u)/t} is acceleration (a)]

If we choose unit force, the value of k is unity, i.e k=1.

Therefore, F=ma

  • Hence, Newton’s second law can also be defined on the basis of above equation as:

Force acting on a body is directly proportional to the product of mass and the acceleration produced.

  • Momentum is the product of mass of a body and its velocity. i.e.

P =mv             where,

[P= momentum, m= mass of the body, v= velocity of the moving object]

Few examples:

  • Let us think about two bullets having same mass, one is at rest and the other is moving at high speed. The bullet at rest is harmless, whereas, other can harm or even kill. This is because of the force in a fired bullet produced due to its acceleration.


  • When a player catches a speeding cricket ball by lowering his hands, he decreases the acceleration in the ball. Hence, the ball doesn’t hurt his hand as the force is reduced.
  1. Newton’s third law of motion:
  • According to Newton’s third law of motion, to every action there is equal and opposite reaction and these act on two different bodies.


When two bodies interact, force applied by one body is equal and opposite to the force applied by the second body.

Few examples:

  • Swimmer pushes water backwards with his hand and feet (action force). Due to reaction force of water on the swimmer, he is pushed in forward direction.


  • When a ball hits against a wall, it applies force (action) on it. The ball rebounds due to the force applied by the wall on the ball (reaction).

Newton’s laws of motion