Momentum

a truck requires a large force to set it in motion when it has a full load than when it is empty. likewise, far more powerful brakes are needed to stop a train then a pushbike moving with the same velocity. when two bodies, a heavy one and a light one, are acted upon by the same force for the same tiem, the light body builds up a greater velocity than the heavy one. the heavy vehicle is said to have a greater amount of momentum than the lighter one.
if we take newtons second law f - ma and replace the acceleration with its definition as the time rate of change in velocity, a = v / t.
f = mv / t
the force is now defined as the change in mv with time.
the produce of mass and velocity is called momentum and we give it the symbol p.
momentum = mass x velocity.
p = m v
where p = momentum measured in kg m s -1
m = mass measured in kg
v = velocity measured in ms to negative 1 square.
if we assume that f is zero, then the change in momentum p must also be zero. thus if there are no external forces acting on an object, the momentum of the object is constant.
momentum is a vector measurement. it is an indication of how hard it will be to stop an object.
the momentum of any object will never change if left alone.