Michelle's Lab Book - Michelle on the physics of Kung Fu
Chris has challenged me to break three pine boards with a single strike. I've seen him break through concrete, so I know it's possible, but I'm a little bit nervous about it. I just can't imagine being strong enough to break through wood. Chris reckons it's all about technique, but he's been training for 20 years and I've only been thinking about doing this for the past two days!
The need for speed
From what Chris tells me, the most important thing is to make sure my arm is moving at the maximum possible speed when it hits the board. This seems kind of obvious, but what worries me is this: I've tried pushing down on the wood as hard as I can and I can't break it, so why will I be able to break it just because my hand is moving?
Luckily, Chris's advice does make sense from a physics point of view. It's all about Force. A force is a push or a pull that can speed something up, slow something down or change its direction. When I move my hand very quickly for a strike, the wood (which is rigid) must exert a force on my hand to slow it down. Because the wood is brittle and does not bend or compress very much, it has to do this very quickly. To slow my hand down in a fraction of a second means there is a very large deceleration and a very large force acting on my hand.
(this is because of Newton's second law of motion) The maximum force the wood can exert on my hand depends on the strength of its fibres. If the force required to stop my hand is larger than the strength of the wood, the wood will break.
There are two cases where it would not be so easy to break something. One is if the material is elastic and can bend or compress, such as a piece of rubber. In this case, the rubber would stretch and slow my hand down gradually over several seconds, which would require a smaller force. The rubber would not break and my hand would bounce off! The other example is less pleasant. If the material is very strong, although the force required to stop my hand is high, the material will be strong enough to produce it. The hand will stop abruptly and would hurt for a very long time. So I'm not going to try to break concrete slabs just yet!