Why does eg a falling stone crate many ripples on a water surface?

In summary, an object doesn't cross the water surface instantaneously, and so the water wave has some spatial extension. When the stone hits the water, it creates a well and displaces water which flows back into the well. The waves aren't water moving outwards, they are water oscillating back and forth, giving the illusion of something moving in one direction.
  • #1
Antti
27
0
So I've studied physics at the university quite a bit and was a bit surprised when I couldn't answer this simple question a friend asked me. When something lands in water why are several ripples created? Like why is the resulting water wave made up of several peaks and valleys and not just one peak, valley or peak/valley pair?

The only possible explanation I could come up with at the time was that an object does not cross the water surface instantaneously. It transfers energy to the water over some period of time and thus the water wave would have some spatial extension because of that. However I suspect that it's not the whole story.
 
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  • #2
Make a sudden movement at the end of a long rope, and you will see a wave propagate even though no part of the rope follows the wave. The rope won't oscillate because it loses its energy hitting the ground, but water will keep moving until friction stops it.

So the waves aren't water moving outwards, they are water oscillating back and forth, giving the illusion of something moving in one direction.
 
  • #3
When the stone hits the water it creates a well. Displaced water flows back into the well, but the energy it has gained from the stone means it 'overshoots' and creates a peak, which then collapses, forming a well.. Each of these motions generates a ripple and saps some of the energy from the centre.
 
  • #4
haruspex said:
When the stone hits the water it creates a well. Displaced water flows back into the well, but the energy it has gained from the stone means it 'overshoots' and creates a peak, which then collapses, forming a well.. Each of these motions generates a ripple and saps some of the energy from the centre.

Thanks, that sort of makes sense. But he might say "well why doesn't the center return to its original state just once and only create one ripple?". Somehow I suspect energy conservation has something to do with it.
 
  • #5
Antti said:
But he might say "well why doesn't the center return to its original state just once and only create one ripple?
Because of inertia and the fact that the restoring forces (which try to bring it into the original state) depend on the displacement from the original state. When the water is moving back upwards it has some velocity, and cannot suddenly stop in the original state, especially since the restoring forces at the original state are zero.

Also note that the disturbance propagates in all directions, from every position that is disturbed. Not just from the the original position, where the disturbance was caused externally by the stone. So you cannot have a single circular ripple propagating just outwards, with calm water inside. Because every point on that ripple is a source of a circular wave front. See:
http://en.wikipedia.org/wiki/Huygens–Fresnel_principle
 
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  • #6
Antti said:
"well why doesn't the center return to its original state just once and only create one ripple?". Somehow I suspect energy conservation has something to do with it.

tell him, "same as why a cork bobs up and down when you push it down" :wink:

(and yes, it's energy conservation … the restoring force is always in the same direction (on the way up), so that's lots of energy when it reaches the start position!)
 

1. Why does a falling stone create many ripples on a water surface?

When a stone falls into the water, it displaces a certain amount of water. This displacement causes a disturbance in the water surface, which then propagates outward in the form of waves or ripples.

2. What factors affect the size and shape of the ripples created by a falling stone?

The size and shape of the ripples depend on various factors such as the size and shape of the stone, the speed at which it falls, the depth of the water, and the surface tension of the water.

3. How do the ripples created by a falling stone eventually dissipate?

The ripples created by a falling stone eventually dissipate due to a combination of factors such as friction, gravity, and surface tension. As the waves travel further away from the point of impact, their energy decreases and they gradually disappear.

4. Why do smaller stones create smaller ripples compared to larger stones?

The size of the ripples created by a stone depends on the amount of water it displaces. Smaller stones displace less water, resulting in smaller ripples, while larger stones displace more water, creating larger ripples.

5. Can the ripples created by a falling stone be used to measure the depth of the water?

Yes, the speed and size of the ripples can be used to estimate the depth of the water. The faster the ripples propagate and the larger their size, the shallower the water is likely to be.

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