By the magic of calculus, we can easily see the relation between
force and momentum for a single object. Let's say the a point mass
is being acted on by a force 
. Then we know its acceleration,
right? It's just 
. So what about momemtum? Well if you
go into hypnosis you might be able to regress to a time a few weeks
ago and then remember the jolting definition of acceleration
Using these two equations, we have
But 
is just the momentum we've just defined, so when I snap
my fingers and wake you up, you'll immediately see that
This is the total net force acting on this one object.
Now let's try to understand two objects.