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Today I left my detachable shower head on the floor and as soon as I turned the water on, the shower head started moving and getting jumpy.

Onto my question- why do hose pipes move and jump when water is flowing through them? I have noticed that usually this effect (I'm such a noob that I don't even know what it's called) is noticeable only after a certain rate of water flow.

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  • $\begingroup$ seems like a case of newton's 3rd law $\endgroup$
    – user243016
    Jul 17 '20 at 15:29
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The water flowing through the hose has mass and momentum. The hose and the shower head are changing the direction of flow, which requires a force applied from the hose/head to the water. By Newton's third law, there will be an opposing force applied on the hose/head. That's what causes it to be "jumpy".

As for the need for a certain amount of flow to see it: The head also has mass and therefore inertia (Newton's first law). So, even though there's some amount of force on the head even at small flows, the force is not enough to (noticeably) overcome the inertia and make it jump.

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  • $\begingroup$ Yes. Any curvature of the hose will cause this kind of effect. To avoid it, the hose has to be perfectly straight. $\endgroup$ Jul 18 '20 at 19:09
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When water is ejected through a nozzle it is accelerated. This results because of the lower pressure of water in the nozzle (Bernoulli's principle) and after ejection (atmospheric pressure). Water pressure inside the hose generates the force and causes the acceleration. According to Newton's third law, an equal and opposite force is applied by the accelerated water. This equal and opposite force is called nozzle reaction and is well known to fire fighters.

It means that equal and opposite forces act on the nozzle. But this means there is unstable equilibrium and the slightest variation causes sideways force. Compare this to compressing a rod along its length. Enough pressure, and the rod will buckle (you could not push on the two ends of a straight string without causing the string to buckle). Unlike a rod, a hose does not resist this sideways force. Hence the difficulty firefighters have in controlling hoses at sufficient water pressure.

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Due to the acceleration of the water in the hose, a force opposite to the direction of the water flow is generated at the end of the hose, which pushes the hose to move in the direction opposite to the direction of the water flow. Of course, the hose will only move when this force is large enough. Since the hose is relatively soft and long, this force cannot push the entire hose, so the end of the hose swings periodically.

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