A martial artist must always be in balance. Kicks, punches, sweeps, blocks, spins, stances, and most everything else require a good balance. For a given object, a balance is achieved when all of the forces are equalized around a central point. A martial artist therefore wants equal weight distributed around his or her center of gravity. (The normal center of gravity for a person is right below the belly button, in the middle of the body.)
Figure 10 shows the classic balance beam problem. Given a beam with a few weights distributed along it, where should the balance be placed so that the beam is level? The distances for each weight X, Y, and Z are a, b, and c, respectively, and they are known, while d, the distance to the balance point, is not. To calculate, the following equation can be used:
Don't run for the hills, this is actually a pretty easy equation, and the fancy E just means add up all the values. As an example, let's provide a few values for our weights, X, Y, and Z.
Y, b = 0.5m, weight = 2.0kg
Z, c = 2.5m, weight = 1.5kg
The equation would be:
The distance d is 1.28 m, so the balance beam would be placed 1.28m from the end of the balance beam's origin (the left side, and yes, this does assume the mass of the beam is not contributing). It's very likely that few people will actually perform this calculation prior to every wheel kick, though. What is important is not that you do the math every time, but that you understand the principles behind the math. Being in balance means that when all of the weight in your body is tallied (like the above equation, but it three dimensions), the answer determines where your center of gravity is. If you were to stand with your feet together and lean forward, your center of gravity moves forward. If you continue leaning, your center of gravity will go beyond the point where your feet can prevent you from tipping. A wide stance, on the other hand, gives more leeway for the center of gravity. Just like a cup with a wide base, a wide stance makes it less likely that you will "tip."
A lot of students have difficulty when first performing a wheel kick. It isn't a trivial kick by any means, but the trickiest part is to stay in balance while spinning. If you consider the balance beam problem above, then the problems the students are facing becomes conceptually clear. If the student is tipping forward, then maybe she needs to lean back a bit more. After all, they have a weight (their leg) swinging around their body. They need to compensate with the rest of their body to keep the balance point in the middle. If you also factor in the momentum of the leg as it spins, you can see that this is actually a non-trivial problem. Not only do you have to stay in balance, but you also have to counteract the forces that occur when an object is rotating. There are few physics professors that would dream of calculating this complex problem, with all its strange, non-uniform weight distributions and non-constant accelerations, but, as in all the previous examples, you don't have to do them to understand how the forces involved effect the kick. The problem of balance can be used on every kick. Imagine that Z is your foot. Although Z is a lot lighter than X and Y, they are still to the left of the center of gravity. If you didn't shift your weight toward X, then you would be off balance. One way to maintain balance is simply to lean back slightly during a kick. That may not sound like an Earth shattering revelation, but this isn't an obvious notion for students. At least this way you know why this works. It also shows that you only have to shift your center of gravity, not that you necessarily have to lean. You could simply shift your hips away from the kick, which would move the center of gravity and allow you to kick upright. Other applications of these principles include everything from punches to throws.
- Serway, R. Physics for Scientists and Engineers. Philadelphia, PA: Saunders College Publishing, Fourth Edition, 1996
- Giancoli, D. Physics Principles with Applications. Upper Saddle River, NJ: Prentice Hall, Fifth Edition, 1998