definition of impact....

When an object colllides with another one (this can be a wall, the ground, or another movable object), there must be a force acting on the original object to slow down its motion. This force arises from collision is the 'impact force', and the time interval during which the impat force acts is the impact time, or simply the "contact time" between the objects .

From Newton's Second Law, the rate of change of momentum is proportional to the force applied. Put it in mathematical terms,

F = (mv-mu)/t
where F is the applied force, t is the time interval the force acts, m is the mass of the object, v and u are the final and initial velocities of the object
i.e. F.t = mv-mu
The term F.t is called "impulse", and F is just the impact force and t the time of impact.

An impulse will bring to a change of momentum.