Free Surely You're Joking, Mr. Feynman! by Richard Feynman Page A

you do get an excellent sense of being _with_ it and _in_ it, and having motivation and desire to keep on–that you’re specially chosen, and lucky to be there.
    So MIT was good, hut Slater was right to warn me to go to another school for my graduate work. And I often advise my students the same way. Learn what the rest of the world is like. The variety is worthwhile.
    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 nip to Prof John Wheeler and said, “Which way do _you_ think it goes around?”
    Wheeler said, “Yesterday, Feynman convinced me that it went 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.”
    After some thought, I finally made up my mind what the answer was, and in order to demonstrate it, I wanted to do an experiment.
    In the Princeton cyclotron lab they had a big carboy–a monster bottle of water. I thought this was just great for the experiment. I got a piece of copper tubing and bent it into an S-shape. Then in the middle I drilled a hole, stuck in a piece of rubber hose, and led it up through a hole in a cork I had put in the top of the bottle. The cork had another hole, into which I put another piece of rubber hose, and connected it to the air pressure supply of the lab. By blowing air into the bottle, I could force water into the copper tubing exactly as if I were sucking it in. Now, the S-shaped tubing wouldn’t turn around, but it would twist (because of the flexible rubber hose), and I was going to measure the speed of the water flow by measuring how far it squirted out of the top of the bottle.
    I got it all set up, turned on the air supply, and it went “_Puup!_” The air pressure blew the cork out of the bottle. I wired it in very well, so it wouldn’t jump out. Now the experiment was going pretty good. The water was coming out, and the hose was twisting, so I put a little more pressure on it, because with a higher speed, the measurements would be more accurate. I measured the angle very carefully, and measured the distance, and increased the