Re: The recoil of a hose when the tap is suddenly opened

Dear RichD:
RichD wrote: Yes. You can think of the hose as a snake, jumping towards the inrushing water. Because the water pressure on the hose "opposite" the opening is unmatched / unbalanced at the instant the hose is opened.
David A. Smith

| > > When you open a tap connected to a hose suddenly you see that the = hose | > > will recoil in the other direction of water flow. Why? | > | > It's due to the bends in the hose. | > | > When the water changes direction a reaction force is created _at | > the bends_ due to conservation of momentum. |=20 | Now reverse the process: the hose is under water, | plugged, air filled. The plug is removed, and the | air is evacuated from the other end. The water | rushes in... does the hose 'recoil'?
Yes. Nothing to do with bends, either. There are no=20 exceptions to Newton's third law.

This is a famous question. Usually discussed as a rotating water sprinker under water, first pushing water out, then sucking water in.
Which way does it rotate, if at all? The answer is not intuitive, and I have the experimental setup to prove it! :-)
Brian Whatcott Altus OK

Interesting. Just a guess, but I think the sprinkler would rotate in a normal manner with the water pushing out, and the sprinkler would not rotate at all with water being sucked in. (actually being pushed in by atmospheric pressure). Am I correct? thanks, Tom

It'll rotate in the same direction regardless. Spraying out, the water takes (say) a left-hand bend with positive velocity, whereas sucking in, it takes a right-hand bend with negative velocity. Same delta momentum -> same force -> same rotation.
Cheers,
Phil Hobbs

Two sources of force -
1)) the pressure is the same inside the hose, but the area of the hose is greater on the outer side of a bend than on the inner side -
F=PA
Therefore the force on the longer (outer) side is greater than the force on the shorter (inner) side - thus straightening the hose.
2) The force of the nozzle has a vector component that lies across the hose section "before" the bend - that component of force also makes the hose end move towards the mean axis.
(It overshoots amd moves back and forth because of momentum)

Yes. Try thought experiment with a bottle.

| > > When you open a tap connected to a hose suddenly you see that the hose | > > will recoil in the other direction of water flow. Why? | > | > It's due to the bends in the hose. | > | > When the water changes direction a reaction force is created _at | > the bends_ due to conservation of momentum. | | Now reverse the process: the hose is under water, | plugged, air filled. The plug is removed, and the | air is evacuated from the other end. The water | rushes in... does the hose 'recoil'?
Well now if the hose were straight (all the way to the reservoir) would the _hose_ recoil or would the reservoir recoil?

Your mental model was like mine. Tom had it right however.
No motion at all when a rotating spray is reverse pumped under water. Annoying but true!
Brian Whatcott Altus OK

There are two major kinds of rotating sprinklers, so there is one correct answer for one type, and three for the other type -
1) My rotating sprinklers are the most common type of rotating type, and they use a jet of water to move a spring-loaded weight on the rotator arm, a weight which is propelled to the side (out of the way of the jet), the momentum of which increments the water jet shaft about its axis. If the weight is stopped out of the way of the jet, the rotation is stopped. ( I have done this many times)
No jet into the weight arm, no weight movement, no rotation.
If you spray out of the sprinkler under water, it rotates. But if you intake through the sprinkler, no jet, no rotator arm movement, and it does not rotate.
It does NOT rotate without the jet striking the rotator arm.
---------
2) A T-bar type of sprinkler, where the spray jet comes out of bar at right angles to the bar and to the axis of rotation to provide force to drive the rotation, spins normally under water when pressurized because of the reaction to the PA force pushing the water out of the tips. If water is drawn into the ends, then there exists an opposite pressure differential at the nozzle from that of the outflow condition, and the PA force moves the intake towards the higher pressure, just as a vaccum cleaner hose moves towards your hand when it is intaking mass. However, the momentum of the water anhd the force differential in the bend at the nozzle provides opposite torque to that of the intake nozzle force, so in the T-bar type you can a) have a short sweep bend and low pressure differential at a small orifice nozzle which makes the bar rotate in the same direction, because momentum overcomes nozzle PA and radius (inner vs outer area) force. b) a large orifice nozzle and large sweep bend which makes the bar rotate in the opposite direction because the nozzle PA force and radius force overcomes momentum c) a bend and a pressure differential and nozzle orifice chosen to have it not rotate at all
or so it seems....

Not much because the water enters [is accellerated] from most all directions.
When it exits all of it only goes in _one_ direction.
See sinks in the Ideal Flow Machine:

Bret Cahill

the end of the hose in the water is pulled into the water by suction, when water reaches a bend in the hose the hose will be pushed towards the outside of the bend

Has to do with pool cleaner dance under water? It bends and twists the hose and its spout at the tube end for pool cleaning.
Narasimham

Yes, a fish can move it's own bag.

Rich Grise wrote in news: snipped-for-privacy@example.net:
Doesn't it depend on the density of the fish? A very dense fish could roll the bag along.

without gripping the bottom of the bag the fish can impart no net force on the side of the bag. neither can he create torque.
About all that can be managed are impulses by colliding with the side of the bag, or causing waves.
Bye. Jasen

most fish are neutrally boyant, hence the approximate density of water.
other non-swimming sea creatures, (snails, crusataceans etc) are denser.
Bye. Jasen

jasen wrote in news:emeo29$53b$ snipped-for-privacy@jasen.is-a-geek.org:
Thanks, captain obvious.