Answer:
The average force times the time of interaction equals the change in momentum
Step-by-step explanation:
The impulse-momentum theorem states that impulse (equal to the average force times the time of interaction) is equal to the change in momentum:

where 
I is the impulse
F is the average force
 is the time of the interaction
 is the time of the interaction
 is the change in momentum
 is the change in momentum
We can demonstrate this theorem, by re-writing the force as mass (m) times acceleration (a):

The acceleration is equal to the change in velocity divided by the time interval:

And by simplifying 
 , we get:
, we get:

which is equal to the change in momentum.