When you first learn physics, you learn of Newton’s three laws of motion:
These three can be applied not only to Martial Arts but to self defense in general.
First Law of Motion
Let’s face it. If you don’t get out of the way of someone attacking you, you’re going to get attacked. Thus, the First Law of Motion informs us that we must do something in order to “change the direction” of the oncoming attack.
There is a zone in which you are protecting. In this zone lies the vital parts of your body and the areas of which you wish to protect from being attacked. By applying the right force at the right angle and the right time, you can deflect almost any attack from this zone and avoid getting hit.
Second Law of Motion
As the opponent accelerates towards you, they are building momentum to apply a force to their attack. The greater the force and acceleration, the less actual mass a person needs to impact pain. Martial Arts teaches us that the speed and force behind your attack can turn even the smallest person into a deadly weapon.
This also applies in the converse when looking at defense. The higher the acceleration of the opponent, the less force it takes to lift a greater mass. Thus from a self-defense stance, if a person is running at you like a maniac, it’s actually defensively better for you if you can catch them off guard. In Tai Chi, we learn to use that momentum to redirect the opponent in another direction and the higher their speed, the easier it is to redirect a larger mass.
Third Law of Motion
One of the key parts of training in Marital Arts is to actually practice hitting something like a punching bag. If you’ve done this even once, you’ll realize that for every physical attack you produce, your body receives an equal impact in return. If you’re not trained to do so, you arms, legs and feet will become very sore from constantly attacking something. Thus it’s important in Martial Arts to train in such things as Iron Body to prepare the body for receiving the impact that is to be delivered.
Some Martial Arts even build their whole defense based on the concept that reflecting the impact of a punch from a less vital part of the body, such as the arms, back into the opponent.
So next time you're practicing Martial Arts, don't just think about how to punch and kick -- consider the Physics behind what you're doing.