Why are manhole covers and ripples round?

Because the circular manhole cover is not easy to tilt, it can better protect the safety of pedestrians and vehicles. The circular manhole cover is mainly used because the length of each diameter passing through its center is the same, so that if the manhole cover is rolled up by passing vehicles, its diameter will be slightly wider than the wellhead below, and the manhole cover will not fall into the wellhead.

The ripples on the water spread around at the same speed, so after a certain period of time, those spread ripples become round. When water particles fall, they move in the direction of wave source besides downward. When rising, it moves upward and is far away from the wave source.

Therefore, every particle on the water surface does not move up and down, but periodically moves up and down along an approximately circular orbit. Therefore, the motion of each particle in a water wave consists of longitudinal motion (the propagation direction of the wave) and lateral motion (the vertical direction perpendicular to the propagation direction of the wave). So the water ripples are all around.

Advantages of round and square manhole covers:

Because the diagonal of the square is obviously longer than the length of each side, such a manhole cover is easy to fall into the well along the diagonal direction of the wellhead when it is rolled up, causing potential safety hazards. If the wellhead is round, the ductile iron manhole cover will not fall into the well. However, in rural areas and cable wells are generally square, which can better prevent rainwater and other liquids from entering.

In the process of water wave formation, when a particle on the water surface falls to form a puddle due to external force, the water under the water surface is pushed aside, and then the surrounding water particles begin to flow to the falling place under the action of gravity, filling the concave part of the puddle, and then making it bulge to form a new concave part around it.