F.4 physics force-time graph

In a force-time graph:

slope = rate of change of force
hence, a horizontal straight line ==> constant force
straight line with +ve slope ==> force increases linearly with time
straight line with -ve slope ==> force decreases linearly with time

area under force-time curve = impulse (i.e. force x time)
Because impulse = change of momentum
the area thus also represents the change of momentum of an object under which the force acts.