impulse
Imagine catching a fast baseball. If you keep your hand stiff, it stings; if you let your hand ride backward with the ball, it barely hurts. Either way the ball's motion is stopped, but by giving the stop more time you soften the blow. That total 'push spread over a stretch of time' is what physicists call impulse — the accumulated effect of a force acting for a while.
Precisely, impulse is force multiplied by the time it acts. For a steady force, impulse J = F times delta-t, where F is the force and delta-t is how long it pushes; its units are newton-seconds (N s). If the force changes moment to moment, the impulse is the area under the force-versus-time graph. Impulse is a vector — it points the same way as the force — and it turns out to equal exactly the change in an object's momentum, J = delta-p (this is the impulse-momentum theorem).
This is why safety design is all about stretching delta-t. Airbags, crumple zones, boxing gloves, and bending your knees when you land all lengthen the time over which you are brought to rest, so the same change in momentum needs a much smaller force. Same impulse, gentler push.
A 0.15 kg baseball arrives at 40 m/s and you stop it. The change in momentum is 0.15 times 40 = 6 kg m/s, so the impulse your hand must deliver is 6 N s. Stop it in a stiff 0.01 s and the average force is 6 / 0.01 = 600 N; let your hand ride back over 0.1 s and the force drops tenfold to 60 N.
Same impulse (6 N s), ten times gentler force — just by taking ten times longer.
Impulse is not a force and not a momentum by itself — it is the bridge between them: a force acting over time that produces a change in momentum. A big force for a tiny time can give the same impulse as a small force for a long time.