Momentum
Momentum is the measure of 'how' much motion a body posses and is of prime importance to Mechanics, Mathematically if we have a body of mass moving with velocity then it's momentum is defined as
The central importance of momentum is due Newton's second law which states or rather defines Force acting on a body to be equal to it's rate of change of momentum.That is if is the net force acting on a body then we have
This means if a body moves such that it's momentum vector remains invariant with time then we can conclude that there is no net Force acting on it