physics of blocking a punch

If a punch is coming at you and you partially block it, but from the side (i.e., your hand or forearm slaps or hits the forearm of the attacker) and the fist continues in to hit you, will this glancing side semi-block, don’t know what to call it, reduce the force of the blow? If so, how is this calculated for the physics-faint-of-heart in a model?

(Assuming that the areas it would have hit had it gone in untouched and that it did hit when you touched it doesn’t matter…just asking about the force.)

I believe you’re looking for how to do vector addition.

In general, the force of the punch is changed very little by a deflection. This is the whole point. If you want to stop or appreciably slow a moving body (punch or anything else) you have to oppose it with a comparable amount of momentum. However, you can deflect a moving body by applying a small sideways force that changes it’s direction slightly, and this force does not have to overcome the body’s existing momentum. The punch still has about the same “force” (momentum, energy, whatever), but the deflection may cause it to miss the target. If not, it may at least hit as a glancing blow in which only part of the punch’s energy is transferred to the target.

Compare to theoretical schemes for using explosives to deflect asteroids on course to impact Earth. Our explosives don’t have the power to stop or destroy the asteroid, but we might deflect it slightly, and if this deflection happens early enough, the new course might miss Earth. If the asteroid still hits us on its new course, it will hit with an impact pretty much unchanged from its original effect. Like daniel801 said, it’s vector addition. A punch is basically a certain mass moving at a certain velocity, and that velocity is a vector. A deflection is a small force acting sideways on the body (fist, wrist, arm) which causes the body to have a new velocity added to its old. The resulting velocity is relatively unchanged but the direction has been adjusted to a more suitable course.