Newton originally wrote his Second Law in a different form, a form that is actually more general than the one we've been using.

SF

=

ma

=

m

Dv Dt

=

D(mv) Dt

We can get away with writing the above equation if the mass is constant.

The equation above has the quantity mv in it - a net force acting on an object over a time period produces a change in this quantity.

Can you think of a good name for this quantity, mv, that is so directly tied to the net force?

We call mv momentum, and give it the symbol p.

General form of Newton's Second Law:

SF

=

Dp Dt

This reduces to SF = ma if the mass is constant.

Turning the general equation around we get:

(SF) Dt = Dp

A net force acting over a time interval is called an impulse.
Impulse is the product of the force and the time interval over which the net force acts.
An impulse produces a change in momentum.
The impulse is the area under the net force vs. time graph.