The following question came up in a swimming pool:

The pool has a jet that releases water below the surface. When I put my right hand in the water, several feet away from the jet, I can feel the pressure from the jet. (I believe that technically, I'm feeling a longitudinal wave created by the jet - is that so?) If I then put my left hand directly in front of the jet, it deflects the wave and I no longer feel the pressure on my right hand. When I remove my left hand, I once again feel the pressure.

When I remove my left hand from blocking the jet, it takes a second or so before I once again feel the pressure. This makes sense - it takes time for the wave to travel through the pool. However, when I block the jet, I stop feeling the pressure on my right hand immediately. Why is this so? I would have guessed that analogous to the unblocking case, the wave already past my left hand would continue to travel for another second, so I wouldn't immediately stop feeling it.

Please help me so that I can swim peacefully once again!


1 Answer 1


What you feel with your right hand is not a wave (which is a propagation of a disturbance), but a stream of water (involving actual movement of material) coming from the jet.

It has some kinetic energy associated with it, but, due to the high viscosity of water, this energy is quickly exhausted, if the stream is not backed up by the pressure from the new water coming from the jet.

To generate a wave in the water, you can, for instance, push it with your hand or drop something in it. Once a wave (disturbance) is created, it moves by itself, so it would be impossible to stop it by inserting an obstacle behind the wavefront.


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