# "Wave motion at the surface of water is made up of small circular motions of parcels of water" True?

Someone I think of as pretty skeptical and intelligent shared this image* with the caption quoted in the title of the thread.

I’m not so sure, but I’m not so unsure either.

Clearly the circles aren’t supposed to be microscopic, or the waves being discussed would not be the waves of water we’re expecting someone to be talking about when they mention “wave motion at the surface of water.”

Also, this is a two dimensional image which creates an impression of waves of thickness but it’s not obvious what this would imply for wave motion in three dimensions.

Yet it doesn’t seem completely implausible.

What do you think? Is this a good illustration of some well known fact?

*I am not sure how to link to the image itself. It’s currently at the top of the linked page.

Sort of true. What that (animated GIF) is, is a 2D representation of a 3D phenomenon. Try to imagine the circles turned 90 degrees so that they stand vertical, rather than horizontal, and are aligned with the direction of wave travel. With a bit of imagination the image gets you there. Then you have wave motion on a 3D fluid.

I guess that image is some sort of perspective view? The actual circles are vertical. As they must be, given that the waves are moving up and down.

Not a really good illustration of something that’s not that well known, but should in fact be obvious. The high school physics description of waves in general focuses on the substance that makes up the wave not travelling and often illustrates using a rope, with points of the rope moving just up and down. In reality even these points won’t move just up and down, and with water it’s even more obvious. A trough needs less water and a peak needs more, so water has to be moving in other directions than the vertical. In the end that motion is cyclical, so the simplification gets the main point across better.

IMO the graphic reffed in post #4 is not physically accurate. It is in other words, an optical illusion. The dots rotate about the circles in a horizontal plane. The rhythmic changes in density of dots gives the appearance of raising waves in the 3rd dimension.

What really happens in surface waves in liquids does involve roughly circular motion, but nothing like what’s shown in the illusion. As such the illusion graphic provides zero insight and is actively misleading as to what’s really happening in liquid surface waves.

The linked image isn’t a very good model of a water wave. But it is true that surface water waves are made up of individual bits of water moving in circles.

This: https://classzone.com/books/earth_science/terc/content/visualizations/es1604/es1604page01.cfm
is a far better animation showing it.

Here’s a better animated GIF viewing a wave from its side. This is a computer simulation of a wave; each particle doesn’t move in a perfect circle, but it does move in a sort of looping fashion.

Note that this isn’t a surface wave, but a shallow water wave, hence, you get the transport phenomena; for a surface wave, the motion would be in a closed circular path.