[Potironette]

I know that if I ever need to answer the question: "How do airbags protect you" for a physics test, I'll just say: "Because while change in momentum in the same, the time it takes for the force to reach you is lengthened, thus reducing the force you receive once it actually hits you (Δp = FnetΔt)"

But what is Δt actually..? Since momentum is conserved, is the change in momentum just to say that if momentum changed, the thing with the momentum most have caused a force to act over a certain amount of time? And so when two balls collide and the momentum has not changed there is no net force happening, but for each particular ball, maybe, the momentum has changed and so it experienced a net force?
Then is "impulse" just a thing which is for specific objects and not for entire systems?
And also, when ping pong balls do collide, since they bounce away with new velocities (and thus new momentums, and I guess accelerating backwards too) I have to assume that time has occurred during their hitting each other? Actually, I guess I should assume that when two things first touch there's no force whatsoever and then it takes time for a net force and changing directions to actually happen o~o?


EDIT: Unrelated
Is work done so long as there is a net force and is net work done so long as there is a change in velocity?