Solving Feynman's Puzzle: Which Direction Does the Tube Spin?

[carlz]

I am not too sure where to post this, but i suspect that it is mechanical so here goes...

Not too long ago, i read a book about by Richard Feynman and came upon this puzzle.

If you twist a metal tube to form a 'S' shape, then pivot the tube in the center. Next connect a water hose to the center of the tube. The momentum of the water spurting out from both ends of the tube will make the tube spin around like a sprinkler, with me so far?

Alright, now the question comes. What if you use the exact same set up, but immerse everything into water. Instead of the water hose, put another hose which leads to a pump, so it sucks in water from both ends of the 'S' tube. Now, how will the tube spin.

Will it spin the opposite direction as the first set up, or will in spin in the same direction.

Feynman proposed really convincing answers for both options but i forgot what are they and what is the final answer. Anyone ??

 

[Danger]

My first thought is that it would spin in the reverse direction because the nozzles are 'vacuuming' themselves through the water rather than 'blowing' themselves along. I'd like to see the arguments on both sides.

 

[Cybersteve]

Okay I'll have a go.

In the original case the "S" would rotate anti-clockwise.

When the flow is reversed I think it will still rotate anti-clockwise because of the way the water flow pushes against the "upright" portions of the "S"

 

[Danger]

Interesting. I never thought of that.

 

[Cybersteve]

Interesting. I never thought of that.

Maybe that's because I'm wrong?

 

[Danger]

Possibly.:rofl:

I really don't know though, so I'd still like to see Feynman's conclusion.

 

[carlz]

alright, give me till the weekend. I will go and try to borrow the book from the library again to tell you guys the answer. Meanwhile, any genius knows it, please contribute.

If you have read 'Surely you must be joking, Mr Feynman', do read it. Its a really inspiring book for physicts. Even if you have no background in science, it is not very technical, just read about his exploits.

 

[Francis M]

alright, give me till the weekend. I will go and try to borrow the book from the library again to tell you guys the answer. Meanwhile, any genius knows it, please contribute.
If you have read 'Surely you must be joking, Mr Feynman', do read it. Its a really inspiring book for physicts. Even if you have no background in science, it is not very technical, just read about his exploits.

I'd be interested in his conclusions as well. Especially his reasoning for why it might spin the same way whether vacuuming or blowing water (I automatically went with spinning the oposite way). I'm currently on "Six Easy Pieces" but I'd eventually like to work my way to the one you listed.

 

[Clausius2]

It would spin in opposite way. This problem has been widely treated here. For solving it:

i) Use the search tool of PF and surely you will find some other threads about this stuff

ii) demonstrate it spins in the opposite way by solving the integral angular momentum over an appropriate control volume of fluid.

 

[carlz]

I didnt quite understand the last part of what Clausius2 said, haha. Anyone care to explain it in simpler terms ?

 

[enigma]

Uhm. WIthout resorting to calculus, he said to add up all the little spinning pieces of water. The parts that will be spinning in one direction will be equal to the parts which are spinning in the other.

 

[carlz]

hehehe, here is the answer from the book :

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 pressure again, and suddenly the whole thing just blew glass and water in all directions throughout the laboratory. A guy who had come
to watch got all wet and had to go home and change his clothes (it's a miracle he didn't get cut by the glass), and lots of cloud chamber pictures that
had been taken patiently using the cyclotron were all wet, but for some reason I was far enough away, or in some such position that I didn't get very
wet. But I'll always remember how the great Professor Del Sasso, who was in charge of the cyclotron, came over to me and said sternly, "The
freshman experiments should be done in the freshman laboratory!"

 

[carlz]

so they turn the same way ! interesting...

Its because of the centripetal force. Ok, just read it yourself, its easy to understand. I hope i don't get into any thorny copyright issues by copying 1 page above.

 

[TheAntiRelative]

In the text you quoted he didn't give the answer of the direction it spun...

He did however relate that he was incompetent in his expectations of failure when putting pressure into a corked bottle... :grumpy:

Additionally, if centripetal force is the only consideration then why do firemen even have to hold onto a straight hose when it's pumping out water?

 

[pallidin]

I think the submerged S-shaped tube suctioning the environments water would not rotate at all.

 

[carlz]

He did mentioned that in both cases the tube will turn in the same direction.

Well, the only way to find out is for someone to actually try it. Too many explanations and all sounds logical.

 

[Clausius2]

Then Fenyman and Wheeler seem to be in contradiction with Navier and Stokes. Who wins? I bet for N&S.

 

[kylemoto]

This question should pose no problem to a junior in mechanical engineering. The "s" does indeed spin anti-clockwise, the same way as when the "s" is supplied with water, and you can quickly prove this to yourself by performing (as previously suggested by Clausius2) a control volume analysis over the "s" shape where the control volume crosses the open ends of the "s" pipe. Or to make the analysis simpler just take half of the "s" shape including only one open end and set your forces equal to the mass flow rate times the velocity at the open end. Both cases will yield a reaction force in the same direction.

detail:
take the upper part of the "s".
Water flowing out of pipe:
the velocity vector dotted with the outward normal is positive, and assume X positive to the right, the mass flow times velocity is positive.

Water being "sucked" into pipe:
Same upper half of "s". The unit normal outward vector dotted with the velocity vector is negative, but the velocity vector is negative (assuming same sign convention as previous example), which yields a positive result of mass flow times velocity - the same result as in the previous case.

Intuitive explanation:
Whenever a fluid passes (subsonically) around a bend the outside wall sees a higher pressure than the inside wall, no matter which direction the fluid is coming from. Look at the "s" shape and study the bends. You can quickly see it will rotate anti-clockwise in both situations.

this is a very nice forum you guys have here, i am glad i found it.

 

[carlz]

I found another interesting way to test it, haven't tried it myself by it sounds logical. Much simpler also.

First, take an empty aluminium drink can.
Punch 1 hole at the side of the can at an angle. The hole will be somewhere near in the middle.
Repeat this at exactly the opposite side.
Suspend the empty can using a string at the top such that it is balance.

Ok, when you pour water into the can, it will rotate clockwise/anti depending on the angle of the hole. This is similar to the first part mention in the thread.


Now for the second part, drop a lead weight into the drink can and place it into a basin full of water. Ideal case, the lead weight should be heavy enough to sink the can where the hole meets the waterline so the can will start to fill with water. There we see which direction it rotates.

Cool, but not tested yet.

Food for thought : Earth revolves around the sun and a spinning top rotates around. So, why is a gun called a revolver and not a rotater since it makes the bullet spin.

 

[Cybersteve]

I've actually got a garden sprinkler somewhere in the bowels of my shed that would be amost perfect for testing this. I was thinking of putting it in the upstairs bath and lowering the hose out of the window, then siphoning the water through the hose.
If the weather gets better for the weekend I might try it.

PS I think a hand gun is called a revolver because the chamber that holds the bullets revolves, I'm not sure if the bullet spins deliberately from a revolver.
It does from a rifle though, due to the "rifling" of the barrel -coincidence? ;o)

 

[Danger]

You're mostly right, Steve. All bullets spin for stability, including shotgun slugs (which have fins built in since shotguns aren't rifled). The cylinder in a revolver rotates on an axle, but the chambers bored into it revolve in an 'orbit' that sequentially aligns them with the barrel.

Carlz, how can you manage to angle a hole through such thin material?

 

[Cybersteve]

You're mostly right, Steve. All bullets spin for stability, including shotgun slugs (which have fins built in since shotguns aren't rifled). The cylinder in a revolver rotates on an axle, but the chambers bored into it revolve in an 'orbit' that sequentially aligns them with the barrel.
Carlz, how can you manage to angle a hole through such thin material?

I'll take your word for it Danger,
I was under the impression that bullets from most revolvers didn't need to spin because being comparatively close range weapons stability wasn't so much of a problem.

 

[Danger]

Some handguns are meant for close-range only, but they still require spin-stabilization. As for others... well, my .44 magnum was fine for hunting something up to the size of a small deer past 100 metres.

 

[carlz]

Wow, i didnt know that shotgun slugs spin too.. It though it was delibrately unstable so they could cover a larger area.

Regarding the hole, punch a screwdriver directly into a can perpendicularly, then you bend the screwdriver to one side such that it is now tangent to the hole punched.. That being said, i haven't had time to try it myself though.

You're mostly right, Steve. All bullets spin for stability, including shotgun slugs (which have fins built in since shotguns aren't rifled). The cylinder in a revolver rotates on an axle, but the chambers bored into it revolve in an 'orbit' that sequentially aligns them with the barrel.
Carlz, how can you manage to angle a hole through such thin material?

 

[Intuitive]

Wow, i didnt know that shotgun slugs spin too.. It though it was delibrately unstable so they could cover a larger area.
Regarding the hole, punch a screwdriver directly into a can perpendicularly, then you bend the screwdriver to one side such that it is now tangent to the hole punched.. That being said, i haven't had time to try it myself though.

I don't know if the wad of the shot will spin in an unmodified shot gun barrel but you can get your Shot Gun rifled at your local Gun Smith shop to fire Slug loader rounds, I use to fire Muzzle loader rounds out of my un-rifled 12 gauge 3.5 inch, 1 shot, Mag winchester. They were accurate up to about a hundred yards then the drop rated was to much after that, You could probably get a couple hundred yards from a rifled version.

A rifled 12 gauge 3.5 inch firing 1-shot slugs with a scope would be fun at the firing range.

In my latter years I prefer Bow hunting now. 85lb Compound Bear.

 

[Danger]

Wow, i didnt know that shotgun slugs spin too.. It though it was delibrately unstable so they could cover a larger area.

The large area coverage, and it's not as large as some people think, is provided by shot cartridges. Those are the little steel or tungsten or whatever pellets (lead is illegal now) that fill a normal cartridge. The slugs that I referred to are hollow lead 'bullets' with angled longitudinal ridges that provide aerodynamic spin. In some cases (Brenneke, at least), the base wad is attached to the rear and accompanies the slug on its full travel.

 

[carlz]

Since we are on this subject. I once saw this weapon which shoots out little sandbags as the projectile. The recoil of this rifle is quite a bit, but the victim being hit by the sandbag is equalvalent to a punch by mike tyson at his peak.

It basically spreads out the energy of the projectile to a larger suface area so that there will be no penetration or servere injury to a single spot.

I suppose it is used by law enforcement agencies who wants to take out a violent personel but not cause any serious injuries other than a blackeye and broken nose.

 

[carlz]

Oh, by the way.

This thread is about interesting stuff, not strictly the S-tube subject. If you guys want to discuss any interesting topics but not too academical, just post it here.

 

[Danger]

Since we are on this subject. I once saw this weapon which shoots out little sandbags as the projectile.

It's generally just called a 'bean-bag' gun. Only the projectile is special-purpose, and can be filled with shot, plastic beads, or as you said, sand. Unless someone is now building something specifically for that use, the standard weapon to fire it from is the M-79 grenade launcher.