If a body receives a push, it's velocity will increase in the direction of the force. It will accelerate in the direction of the force and will receive an impulse. The impulse
is equal to the change in momentum and we can write
It is important to take the direction of the velocity into account.. It is best to fall into the habit of taking up and right as positive, and left and down as negative.
Example:Below are before and after diagrams showing a collision between two particles.
The impulse exerted on the 5Kg mass is
The negative sign mean the impulse acts to the left.
The impulse exerted on the 7Kg mass is
The positive sign means the impulse acts to the right.
Notice the difference in sign. This is a consequence of Newton's Third Law: Action and Reaction are equal and opposite.