The science for this one is not too tough, but it does seem to catch a lot of people off guard, so I figured I would turn it into a short video:

It turns out that punches are not like projectiles. Your muscles make a trade-off between adding to the velocity of a strike or adding to the effective mass, and this trade-off gives us a dichotomy based on quantities that we usually think of as changing together in simple classical mechanics scenarios.

If you want to get into the details, it is an inc...