>>11481646because "ma" is the product of acceleration times the mass of the thing that is accelerating.
when you punch, only your arm and fist move. the total impulse (the integral of the force your fist imparts) is basically equal to the mass of your fist+wrist+a certain proportion of your arm * its impact velocity divided by the time it takes to grind to a halt after impacting the thing. (in other words, if it stops instantaneously, like hitting a brick wall, the force is very large for a very small time, that's why punching a brick wall makes your knuckles bleed, whereas punching a person doesn't make your knuckles bleed because your fist slows down over a more gradual time interval thanks to the relative softness of a person)
i probably mangled this explanation but if you want clarification i will try to do better
