|I once did an experiment in the cyclotron laboratory at Princeton that had some startling results. There was a problem in a hydrodynamics book that was being discussed by all the physics students. The problem is this: You have an S-shaped lawn sprinkler—an S-shaped pipe on a pivot—and the water squirts out at right angles to the axis and makes it spin in a certain direction. Everybody knows which way it goes around; it backs away from the outgoing water. Now the question is this: If you had a lake, or swimming pool—a big supply of water—and you put the sprinkler completely under water, and sucked the water in, instead of squirting it out, which way would it turn? Would it turn the same way as it does when you squirt water out into the air, or would it turn the other way?|
The answer is perfectly clear at first sight. The trouble was, some guy would think it was perfectly clear one way, and another guy would think it was perfectly clear the other way.
So everybody was discussing it. I remember at one particular seminar, or tea, somebody went up to Prof. John Wheeler and said,
Which way do you think it goes around?
Yesterday, Feynman convinced me that it goes around backwards. Today, he's convinced me equally well that it goes around the other way. I don't know what he'll convince me of tomorrow!
I'll tell you an argument that will make you think it's one way and another argument that will make you think it's the other way, OK?
One argument is that when you're sucking water in, you're sort of pulling the water with the nozzle, so it will go forward towards the incoming water. But then another guy comes along and says,
Suppose we hold it still and ask what kind of a torque we need to hold it still. In the case of the water going out, we all know you have to hold it on the outside of the curve, because of the centrifugal force of the water going around the curve. Now, when the water goes around the same curve the other way, it still makes the same centrifugal force toward the outside of the curve. Therefore the two cases are the same, and the sprinkler will go around the same way, whether you're squirting water out or sucking it in.