Thursday, May 15, 2008

Olympic-Sized Physics (Part one)

There is very little I enjoy more than sitting on the couch eating potato chips and watching the Olympics. Not only do I get to revel in the fact that, not being a world class athlete, I can eat such things as potato chips, but I also get to see tons of physics in action. Really in action.

With the Olympics coming up I felt it might be nice to have some posts on the physics of Olympic sports. I am biased and unapologetic about it. I am a swimmer by training, a cyclist by default and a runner by necessity. So those are the sports I'm going to talk about in this series.

The physics of swimming is a very involved subject (for a comprehensive guide, see Swimming Fastest by Ernest W. Maglischo) so it is easiest to focus on one aspect, and in this case we will look at the construction of a pool. You may hear announcers describe pools as being "fast" or "slow" but what does that mean? Isn't it all just the same water? The way the pool is designed can mean the difference between a world record and an average performance. When designing a pool there are five main things that need to be optimized for speed, the temperature, wall material, depth, gutter style and water recirculation. The temperature is the easiest and is regulated by FINA, International Swimming Federation, to be 77-82.4°F. This temperature is not too cold so the swimmers don't waste energy heating their bodies, but not so warm that they overheat. The surrounding air must be kept within 3°F of the pool temperature. The wall should be made of a stiff material, like ceramic tile, so that swimmers get the most out of their push off the wall. If the wall is springy some of the energy of the push will be used to bend the wall, not move the swimmer forward. Is it a huge energy loss? No, but when 0.01 seconds is the difference between a medal and defeat, you take what you can get.

The remaining four variables all deal with how much wave interference a swimmer must endure. When waves hit a swimmer they slow him down. Think of trying to run into the wind. You may be using just as much energy to go forward, but the speed of the wind pushing back on you means that you are actually going slower for the same amount of work. For the physics students among you, you are adding the velocity vector of the runner and the velocity vector of the wind and the resulting velocity vector is slower. If the swimmer is unlucky, the waves also shoot water up his nose and make him cough, which for an Olympian is just embarrassing.

When a swimmer takes a stroke he both pushes back and down. The push downward causes a reflection off the bottom of the pool. If the pool is shallow, that reflection bounces right back up and hits the swimmer. If the pool is deeper it takes longer for the reflection to reach the top again and the swimmer has already scooted on by and won't be affected. The sideways waves produced head toward the walls and will reflect off of them. The person swimming in the middle lane is said to be in the "fast" lane because the wall reflections hit all the other swimmers before they get to him. If the gutter is designed well the water just spills on over into the gutter and doesn't reflect back so the guys in the "slow" lane don't have to worry as much. Last is water recirculation. If the water being dumped in the pool just shoots out the side, the guys in the side most lanes have a rough job. Getting hit in the face with a water jet is never fun but it can be particularly annoying when you are the underdog in the race of your life. No one wants to swim against the current, its tiring and you never get anywhere. The fastest pools squirt the water back in slowly from jets at the bottom. If the water is put back in slowly it pushes against the swimmers less. The bulk of the art of pool design is in reducing the size and velocity of the waves hitting the swimmers. It will probably surprise no one that MIT has one of the fastest collegiate pools in the US. I'm sure some first semester freshman designed it for a homework assignment. Silly, overachieving Beavers.

I hope you enjoyed learning a little more about the swimming pool. At least I gave you some good excuses for losing your next race, "the sun was in my eyes, no wait, I meant the pool was too slow!"

1 comment: