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In class we discussed an experiment for measuring the speed of waves in water. The basic procedure went something like this:

Make a wave at the near end of the tank and wait for it to travel to the far end. Start the stop watch. Time the waves doing several "laps" between the near and far end of the tank. Then stop the stop watch and calculate the speed based on the time taken, the length of the tank, and the number of bounces.

I understand that we want the waves to bounce from the tanks ends a number of times to reduce the percentage uncertainty in the time measured. But that got me thinking do water waves take a certain amount of time to reflect? Intuition suggests they do take some time or slow down, however thinking about the situation I see no reason why the changing of direction shouldn't be instantaneous, the water is moving up and down anyways.

Is my reasoning that waves bounces should be instantaneous correct? Or are there other even slight effects at play which I didn't consider?

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  • $\begingroup$ The bouncing will be (virtually) instantaneous. The reason you need to count the number of bounces is because the total distance the wave has travelled with equal the length of the tank multiplied by the number of bounces (plus one). $\endgroup$
    – lemon
    Commented Mar 26, 2015 at 19:00
  • $\begingroup$ Sorry if i phrased the question wrong. I meant to ask will the bounce be instantaneous? What factors (even small ones) might make it non-instantaneous? $\endgroup$
    – Michal
    Commented Mar 27, 2015 at 0:39

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Water waves are made of many molecules, which attract each other, so returning molecules, having taken time to turn, are also slowed down by their neighbors, and, having rest mass, are affected by gravity.

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