Fight Science is an entertaining show. Great graphics. The basic idea is to look at the science in different fighting styles. They had a clip-style commercial on it during a MythBusters episode I was watching. And from that, I can say that the kicking looked cool, but the science needs some work.
The basic idea is that they wanted to compare kicks from different fighting styles. From what I can gather, they collected data by having some dudes kick this “kicking bag”. During the kick, they measured the force exerted on the bag and they had a sensor on the kicker’s leg – I assume to measure the kinematics of the kick (position, speed, acceleration). I didn’t get (nor could I find online) a good picture of this setup, so I will use my uber-drawing skills instead.
Here is the problem. How do you physics-ly determine which fighting style has the most awesome kick? I can think of a couple of things, but Fight Science focuses on speed (or acceleration – you can judge) and force. I tried to get a basic transcript of the important part. Here it is (paraphrased since I couldn’t clearly hear the names of the kickers). I think I got the important things they said. (I used letters to represent the different kickers)
“Which kick was the most powerful? C’s strike produced the highest ration of force to velocity. It was the most effective technique, but why? The answer: physics. The force of the kick relies on both mass and acceleration. C wasn’t the fastest, only 99 mph to Taekwondo’s 136. But, Capoeira delivers the highest ratio of mass to acceleration in a single strike. From there, it is simple mathematics. And the numbers never lie“
I am not making this stuff up. That is just about exactly what was said. There are a couple of huge errors here – and that is what I will focus on.
Suppose I wanted to compare boxes of cereal – you know, just because. To make this comparison, I measure the volume of the box and the mass. I do this because clearly those are both important things. What to do now? I have two variables for each box. How do I compare them? Oh wait. I know, I will divide the mass by the volume. So, here are the results.
I left off the units because I just made this stuff up anyway. The best cereal is box C. Although Box A has the biggest mass, its ratio of mass to volume is smaller making Box C clearly the best.
Why are they dividing force by velocity? I don’t get it. Oh, wait, it’s physics. Here is the data they presented.
Acceleration and Velocity
I hate to sound so attacky, but I just don’t get it. At one part they say velocity. On the graph, they present the speed in miles per hour. And then they try to relate this to acceleration – you know because of Newton’s second law. Acceleration and velocity are not the same thing. Velocity is the time-rate at which something’s position changes. Over some time interval interval, the average velocity is:
Acceleration is the time-rate at which the velocity changes. Over some time interval, the acceleration is:
Not the same. For the same motion, one can be zero, and the other can be non-zero.
Newton’s Second Law
I don’t know where they (Fight Science) was trying to go, but it seems like they are talking about force, mass and acceleration. They probably wanted to quantify the kick in terms of Newton’s second law, which is commonly written as:
So, if I am correct that they are using velocity as acceleration, what does their ratio of mass to acceleration mean? Maybe they meant to say something else. I am having trouble grokking this whole thing. Maybe they are also confusing force and mass. They didn’t measure the mass, so how could they find the ratio of mass over acceleration?
The Real Problem
The mistakes aren’t the problem. The problem is the goal. It seems that they are trying to use this kicking exercise as an example to talk about physics. They have fallen in the classic trap of throwing out cool physics diagrams and terms that most people would interpret as physics. Fight Science is not alone in this error, lots of other shows do it also (see Ruff Ruffman). I see two better options:
- Option A: Redo the physics and explanations so that they are consistent with scientists’ ideas and they make sense. I know that sometimes this really isn’t feasible to explain complex ideas in a short time frame. If you want to follow this option, feel free to call me. I would love to help.
- Option B: Forget the physics. Just look at the cool kicking stuff.
Note: ScienceBlogs is now a partner with National Geographic. I don’t think I should let that stop me from doing what I normally do though – right?
Note 2: I still think National Geographic is a great channel. In fact, since we are sort of related to them, I should offer my services. I have tons of National Geographic-type skills. (cave diving)