I am sure I have mentioned the Discovery show Time Warp before. The basic idea of the show is to find how many cool things they can look at with an ultra slow motion video camera (btw – this is on my christmas list).
So, I have seen this Samurai guy on the show twice. Yes, he has a big sword. Yes, he could utterly destroy me if he wished. However, I can not handle him talking about physics. Mr. Samurai guy, please stop using the words “kinetic energy” until you know what they mean (I said please). Here is my first problem. Samurai guy is showing how to chop people down. He plans to demonstrate on one of the Time Warp guys. Here is his explanation:
What I am going to do is allow a wave of kinetic energy to pass through my body and into yours and you will feel the way…
What does he mean here? I think he is just trying to make it sound like he knows what he is talking about. People do this all the time. If you use the words “kinetic energy”, clearly you know what you are talking about. (If Mr. Samurai guy reads this blog, please realize that everything I say is a joke.) If you want to watch the video, I found it on youtube (may not be there forever):
The good stuff starts around 5 minutes into the video.
I am not sure if the guy thinks he is a jedi or something, but he pushes the Time Warp guy down by pushing on his head. The time warp guy clearly crumples (as would I).
I think this is a fairly simple trick. He basically moves the body back a little bit so that it is no longer over the feet. Then when he pushes down, there is absolutely no way to not fall down. You can sort of see this in the above picture. Notice how the time warp guy (sorry, I don’t know his name) has his head back a little? Now when samurai pushes down, there will be a torque to cause the guy to rotate. Here is my simplistic diagram:
First, notice that TW guy sticks his arms out. This is an attempt to move his center of mass back over his feet so he won’t fall back. With the samurai guy pushing him down, his arms are not heavy enough to prevent him from falling over. He would need his center of mass far in front of his feet so that the net torque about his feet is zero. It doesn’t matter how strong you are. If there is a net torque about his feet, he is going to rotate (thus fall). Well, I guess if he was spider-man, he could use his feet-to-floor-sticking ability to prevent him from falling over.
The samurai guy keeps talking about energy. I do not think that word means what he think it means. Yes, energy is involved in this situation, but I think forces and momentum are much more important. If TW guy received a bunch of energy from samurai guy, wouldn’t samurai guy have a big decrease in energy?
Now for the super samurai punch
So, this is the “zero distance” punch or something like that. Basically samurai guy knocks TW guy over just holding his hand a couple of inches away. See this picture: (at about 6 min into the video)
TW guy asks a great question:
TW guy: “from a Newton’s law perspective, where is the reaction? If you are giving me that force, how is your body not moving in the opposite way?”
Samurai guy: “It just goes right through you.”
Er? Goes right through you? Ok, obviously samurai guy does not have a degree in physics. Also, forgive me TW guy (because I like you), but he does not “give you a force. Maybe you could say “gives you momentum”, or perhaps “gives you energy”, but force is an interaction between two objects. It is not something to give and take. Ok – back to super samurai punch. How does it work? Not really sure, but it seems that it is sort of like a collision. If you notice samurai guy right before the “punch”, he is moving forward. He uses this forward momentum along with the punch to make the other guy bounce back. It probably also helps that TW guy has his feet right under him (in a normal-person stance) so that if samurai guy pushes him back enough on the top, he will again be in a situation where his center of mass is beyond the support of his feet and he will fall.
I was considering doing a video-analysis of the super punch, but decided not to. Momentum will not be conserved because samurai guy is pushing on the floor with his feet and he is pushing on the TW guy with the punch. The net effect is that the samurai guy slows down and the TW guy speeds up. There is a net force on the system of the two guys from the friction due to the floor. Maybe I can show this with a picture.
Maybe that doesn’t add to much to the discussion, but oh well. Really, it is not the samurai punch that bothers me as much as the “energy flowing” stuff. Once again Mr. Samurai guy – I am just saying physics stuff. Please do not come after me with your super punch or your sword. Didn’t the MythBusters test the 1 inch Bruce Lee punch? I can’t remember.