The momentum of a body is the product of its mass and its velocity,

for a body of mass m, moving with velocity v,

momentum = my

Because momentum is a scalar multiple of velocity, which is a vector, it follow that momentum also is a vector quantity.

When the velocity of a body is constant and its mass does not change, its momentum is constant.

We know that a force is needed to change the velocity of an object and it follow that a force must act on the object in order to change its momentum. The precise relationship between a force and the change in momentum that it produces be found by combining Newton's Second Law with the equations of motion with constant acceleration.

Consider a constant force F that acts for a time t on a body of mass m in the' direction of its motion, causing the velocity to increase from u to v. As the force is constant, the acceleration a that it produces, is also constant.

Using F = ma and v = u + at gives

               v = u + t (F/M)

   => Ft = mv – mu

So we see that the change in momentum, i.e. final momentum minus initial momentum, is given by the product of the force and the time for which it acts.