Linear Momentum: Relation with 2nd Law of Newton

Definition of Momentum

The momentum of a body is the quantity of motion it has due to its mass and velocity.

We understand the fact that moving item possesses a quality by means of which it puts in a force on anything that tries to stop it. The faster the object is moving, the more difficult is to stop it. Similarly, if two items move at the exact same speed, then it is more difficult to stop the massive of the two.

This quality of the moving body was called the quantity of motion of the body, by Newton. This term is now called linear momentum of the body and is specified by the relation.

Mathematical Form

Linear momentum = p = mv

ln this expression, v is the speed of the mass m. Direct momentum is, therefore, a vector quantity and has the direction of speed.

SI Unit

The SI unit of momentum is kg metre – per second (kgms-1). It can likewise be expressed as newton second(Ns).

Momentum and Newton’s Second Law of Motion

When a force acts on a body, it produces an acceleration in the body and will be equal to the rate of change of momentum of the body.


Consider a body of mass m moving with an initial velocity vi. Suppose an external force F acts on it for time t after which velocity ends up being vf. The acceleration produced by this force is given by

                 a = vf – vi / t

By Newton’s 2nd law of motion acceleration, a is

                            a = F/m

Comparing the two values of acceleration

F/m= vf – vi / t


F x t=mvf -mvi


  • m vi –> initial momentum
  • mvf –> final momentum

The change in momentum amounts to the product of force and the time for which force is applied. This kind of the second law is more general than the type F =ma since it can easily be extended to account for changes as the body accelerates when its mass also alters. For example, as a rocket speeds up, it loses mass because its fuel is burnt and ejected to offer higher thrust.