distance away from the rotational point.
Torque [NM] = Torque equals Force times Distance.
For instance, say you are applying a joint lock on the thumb. Say you press at the near side of the thumb nail, 1 cm from the joint. Then say you instead press at the far end of the thumb nail, 2 cm from the joint. By only moving one centimeter away, you have effectively doubled the Torque on the joint and doubled the pain of your opponent.
The best way to cause rotation is to apply two forces such that they add together and both put Torque around the part of the body you want to rotate. Two forces acting in opposite directions is called a Couple. This is extremely important when trying to trip or sweep an opponent. Try to ‘sweep’ someone by putting your leg behind their leg and pushing on their shoulder, and then try the same thing while sweeping with your leg and pushing on their shoulder at the same time. Also, the farther both forces are from the point about which you are rotating, the larger the Torque. This is why can sometimes be easier to trip tall people, or why you are more stable in grappling when you spread out to a low and wide base.
This talk of tripping leads us to the next important concept, Balance. In order to be balanced, the integral (sum) of all the forces acting on your body must resolve to a vector that passes through some point on your body that is on the ground. Sound complicated? It’s not. It merely says that if you lean one part of your body in a certain direction, in order to not be stable you must lean another equal part in the opposite direction. Thus, when doing a side kick, your chest leans back while your foot protrudes outwards. When doing a spin hook kick, your chest leans down and away from the kick to counterbalance your leg.
Angular Momentum and Spinning Kicks
Angular Momentum (the tendency for something to keep spinning) is an important concept for spinning kicks. Angular Momentum is also conserved as long as no forces act on the system. This means that your rate of rotation multiplied by your moment of inertia about the axis which you are spinning will be constant. Or, in another way your shape determines how fast you spin.
The moment of inertia is the integral of each part of your body times the distance away from the axis you are spinning on. So, the farther your body parts are from your rotation axis, the larger your moment of inertia, and the slower you will spin. You’ve all seen ice skaters who start with their arms out (high moment of inertia) and then slowly move their arms in (low moment of inertia). What happens? They start spinning faster.
How can we apply these concepts to Taekwondo? Well, remember the importance of acceleration. Thus, you can change your speed quickly by starting with your arms and legs far out and then once you start a spin you can tuck them in quickly to get an extra boost. For instance, from open stance say you throw a butterfly kick with your legs and