I have noticed that water, when falling will rotate. Looking closely at a thin stream from a faucet and placing a flat object mid stream you will see the water is rotating. The further down the stream you go you will notice the rotation is much quicker. Am I confused at this occurrence or is there a reason? I can only come up with the rotation of the earth taking into affect.

  • $\begingroup$ I doubt that it has anything to do with the rotation of the Earth. $\endgroup$ – user42733 Mar 31 '14 at 15:39
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    $\begingroup$ Water does not spin coming out of my faucet. Assuming your faucet causes water to come out rotating, conservation of angular momentum would cause the water to rotate faster farther down the stream because the stream narrows farther down. $\endgroup$ – Brandon Enright Mar 31 '14 at 15:42
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    $\begingroup$ Yes, the speed of rotation will increase because the stream becomes narrower. No doubt about that. What causes the water to rotate is difficult to guess. Could be the shape of the opening of the faucet. $\endgroup$ – user42733 Mar 31 '14 at 15:45
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    $\begingroup$ It is very difficult to make a faucet, or pour from a cup, without imparting some rotation to the water. As @Parth said, it will rotate faster as the diameter of the stream decreases. $\endgroup$ – Mike Dunlavey Mar 31 '14 at 15:51
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    $\begingroup$ I think the spin is caused by upstream bends in the water piping (slanted bends plus gravity). $\endgroup$ – Mirc Breitschuh Mar 31 '14 at 19:15

I think the simplest answer to this question would be that the stream of water has a number of forces acting on it (gravity, air drag) from many directions. Some torque is bound to be produced as the stream falls through the air. If you throw a ball or any small object from a height, it rotates, no matter how you drop it. Same logic applies here.

As far as the speed of rotation increasing as it goes down is concerned, the reason has to be that the stream is getting narrower, so it has to rotate faster to account for the decrease in the radius due to Conservation of Angular Momentum.

  • $\begingroup$ I think you can improve the last sentence. Angular momentum is not lost. $\endgroup$ – user35033 Mar 31 '14 at 17:08
  • $\begingroup$ It says 'to account' for the loss in angular momentum. The increase in velocity is compensating for the decrease in radius. $\endgroup$ – user42733 Mar 31 '14 at 17:12
  • $\begingroup$ Exactly, you mean that the velocity increases to account for the change in radius. Angular momentum isn't lost. You're right. Just change the phrasing. $\endgroup$ – user35033 Mar 31 '14 at 17:25
  • $\begingroup$ I've dropped many objects that don't rotate for drop tests at precise angles... What would cause a torque of significant magnitude to cause an observed change in speed? (gravity acts on the center of mass, and for nearly symmetric objects the air drag would similarly act near the center of mass) $\endgroup$ – Rick Feb 3 '15 at 17:26

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