In one of the biographies of Richard Feynman, he told the story of trying to solve a thought problem by dint of actual experimentation. It was a funny story, but he never gave the answer!
Here is the thought problem:
Picture a hollow S-shaped tube, with a flexible joint in the middle, perpendicular to the surve of the S. If you pump water through the joint into the tube, it will come out the ends of the S. Pump it with enough pressure, and (because the joint is flexible) the S-shape will start to spin backwards, like a lawn sprinkler.
Now take this same apparatus and put it underwater. Instead of pumping water out, create a vacuum, and suck water in. Which way will the S shape turn?
He gives two arguments, one for each answer.
I don’t wish to poison the well, so I won’t give either of his arguments here.
Can I make a guess before the smart people get here?
It turn backwards since the inertia force is on the outside of the curve no matter which direction the water is going, in or out.
it would spin backwards because the sucking of the water creates a vacuum, and for every action there is an equal but opposite reaction, so it would spin backwards.
It would be easier to show in a diagram, but I can’t really draw anything, now can I?
by the way, JimB, we’re all a part of the teeming millions…Cecil’s the “smart person,” if there, indeed, is only one. So feel free to make guesses as often as you like.
Yeh backwards the same way as if it was squirting. Feynman’s a small thinker anyway. Allatime wantin to make things little. Even the $250,000 dollar prize is worth more than that.
“Pardon me while I have a strange interlude.”-Marx
Actually, forward was my guess, but it seems so obvious that it must be wrong.
To be clear, by forward I mean that the openings at each end of the S would be pulled forward, in the opposite direction from squirting water through it.
Never attribute to an -ism anything more easily explained by common, human stupidity.
It does nothing. This is why you don’t need to anchor the fill side of a siphon while the empty side would wip around under sufficient pressure.
When you drain a fishtank for example, the inlet will remain still unless it is near an obstruction (like a pebble) that impedes the water flow enough to create a variance in pressure, in which case the inlet if it is not anchored will be drawn towards the low pressure zone (pebble.)
As you read this a column of air the height of the earth’s atmosphere is pushing down on you. The reason you are not crushed like a bug by this is because it is also pushing you in all other directions including from the inside out. The pressure is exactly equal and you feel nothing.
Such is the case inside Feynman’s tube once it is filled with water. The pressure is exactly equal.
Prove me wrong and we will all be instantly crushed by the weight of the earth’s atmosphere!
Are you saying that, even under water, Feynman’s tube wouldn’t spin backwards if it were squirting out more water?
I don’t buy that at all.
I admit that parts of this thing defy my ability to reason, but that isn’t one of them. I am convinced that, under water or not, if the tube is squirting, it will spin backwards.
So why is the pressure “unequal” in that case but “equal” if the tube is sucking water in?
I only changed my name 3 days ago, and you’re the first to make that reference in spite of the fact I’ve been “trolling” for it. Thanks for rising to the bait.
Here’s why we would implode if I were wrong:
It is true that there is is an equal and opposite reaction to every reaction. The mistake made here is in assuming that that reaction would be focussed at the inlet.
In fact, the reaction is that the water level is dropping. In turn it is dropping because it is being pushed down by a column of air equal to one atmosphere.
But I don’t understand your reasoning at all. What does the air pressure on the surface have to do with anything? Why isn’t it a factor if the water is squirting out?
Why does the thing move for the squirting but not for the sucking?
meant equal and opposite reaction to every action.
The tube is not squirting. It’s not doing anything. The pump or siphon is drawing water through it. The energy (action) is being expended at the pump in the former case, and the reaction is at the outlet.
I’m going to jump back in here. Whether the tube is in the air or in the water, all the pressures cancel each other and the tube just sets there. The only pressure difference is from the middle of the tube to the ends. The only thing this will cause is the water to flow through the tube. The only thing that will create any motion by the tube itself is movement of the water.
Newton had a law of motion that that dealt with a body in motion tends to continue in the same direction unless another force is applied. The water will try to go straight, therefore banging it’s head on the outside of the curved tube. So, for the water to move along the curved path the outer wall of the tube has to push it out of it’s straight line. For every action there is an equal and opposite reaction, (I didn’t make that up, somebody said it, Newton I think) so,if the water makes a left, then the tube makes a right.
It doesn’t matter which direction the water is going, just that it isn’t allow to go in a straight line.
Jim, are you saying that the tube doesn’t move in either case? What is the essential difference between the cases that makes the tube move when it squirts but not when it sucks? It seems to me in both cases the water is going around the curve in the tube.
There is no net directional force in the S tube once it is filled with water. Take an empty garden hose and turn the water on. If you are holding the hose in a coil you can feel it torque as the water moves through it. Once it’s filled all the force is at the outlet, and if there is a sprinkler head thingie on it, the thingie will spin.
There is no net force in the S tube when filled because once the water gets the through the S it pulls the water behind it
