## Thursday, April 15, 2010

### Swim Rule Logic

I went swimming yesterday and was sharing a lane with another Medium level swimmer that was using flippers. As soon as the life guard saw the wide splashes he was making with his flippers, she ordered us to circle in the lane instead of splitting the lane. Usually two swimmers would split and each swim in his/her column while three or more swimmers would circle. I thought at first, of course it makes sense - we will pass each other much less when circling. But then I got doubtful. What do you think? Do you see any logic in this swimming rule?

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Kim said...

Yes, there is logic to it. If you are circling, then there is relatively little passing, but if you are splitting a lane, there is guaranteed to be a point on each length of the pool (regardless of your relative speeds) at which you are at the same point in the lane.

Anonymous said...

You'll pass each other just about as often (assuming all speeds remain the same); but the near-collisions are avoided by keeping to the "outside" of the lane. That is, if I understand "circling" correctly -- staying to the right side of the lane. Yes, it hink I've got it right, and it is logical and safe. Tom

Maria said...

We were circling but we still crossed each other once every lane. Exactly how we would cross when splitting... I do not see how to avoid it. Only waiting at the end of each lane for the other to catch up.

Anonymous said...

I think circling would be a better choice. Because, if you split lanes you will definitely get splashed at one point but if you are circling and if you stay at a safe distance behind the person who is wearing flippers you will never get splashed while swimming.

Maria said...

I am still not convinced that circling is of any advantage. Of course, if you split and swim in the same direction, you will always interfere with each other. But splitting swimmers always know to keep in opposite directions.

The only way how this may make sense is if (as Tom suggested) the word "circling" means "keeping as close as possible to the right border of the lane", instead of "swimming in the right half of the lane" as in "splitting". Then swimmers will be further when crossing each other in circling than in splitting. Can't give Tom another puzzle point as he already got one this week for the Tile puzzle,

Here is the sketch summarizing all the swimming options.

Other thoughts on this puzzle are still accepted.

Kim said...

After just finishing watching a season's worth of swim team practices, I definitely think of circling as each swimmer using the whole lane, and always staying to the right-most border of the lane.

Maria said...

Unbelievable! My son is also a swimmer. I wonder if we spend our weekends at the same swim meets around MA...

Maria said...

Here is what a friend - Professor at Cornell - posted on Facebook in response to this swim puzzle:

I swim with flippers and in our pool we always split, not rotate, when the difference in speed is large. Rotation only works well when the swimmers are about evenly matched. However... splitting makes you encounter each other on each of your two sides (alternating left and right), whereas rotating does not. For unprofessional swimmers like me, who always breathe on the same side (I breathe to the right), this may be problematic, if they tend to veer toward their blind side.

Steve Bayle said...

IMHO this is more of a psychological problem than a math or logic problem. I've been swimming for years and run into circling at my club when the pool is crowded and you have to share the lane with more than one person. I detest circling as it only works on two conditions: both swimmers swim at the same speed and neither stops to rest. I find this very rare and it is hard to enjoy swimming when you are worried about either catching up to the other person and having to slow down or vice versa. Lane splitting gives you your own lane and you can speed up, slow down, stop and rest all without worrying about the other person. I must admit that all my circling experience involves 3 people sharing not 2 and I don't like sharing a lane with people who use flippers as they do splash. But I think this problem is more one of personal preference and has more factors to it than simply how often you get splashed. Finally how do you know how fast the other person will be swimming while you are in the same lane? How do you know if they stop to rest every 5 laps?