# How does a little stone thrown on the water surface generate a wave?

I'm really interested in the process details of the creation of a wave.
I wonder what exactly happens as soon as the stone hits the surface.

1. Why is a wave generated? Is it due to water molecules pushed down, or pushed apart? (or both of them?)
2. As soon as it touches the surface is generated a crest or a trough? (or both of them?)
• 1) displacement is the driving force and surface tension and pressure the restoring forces; 2) both, trough below rock and crest near outer edge. – honeste_vivere Aug 21 '17 at 14:03

Here is what I know:

1. A wave generated in water is said to be both, transverse as well as longitudinal. Hence vertical and horizontal motion of water molecules, both generate it.

2. If your stone is an ideal point sized mass then it will create a trough. However, a practical stone will create a trough at center and crest around the submerged part of stone simultaneously (approx).

3. It forms a circle because the wave propagates in all directions on the surface of water with same speed.

• The first point is incorrect. Fluids cannot have transverse waves. – Aaron Stevens Aug 22 '18 at 10:23
• @AaronStevens Surface tension can allow them to have transverse waves. – Prasanna Aug 22 '18 at 11:03
• @Prasanna Sorry I was thinking about waves through the medium, not on the surface. You are correct. This should be addressed in the answer. – Aaron Stevens Aug 22 '18 at 11:05

There are two types of wave that will be generated when a stone is dropped into a body of water.

A gravity wave is formed when the falling stone strikes the water surface and pushes aside the water that is directly in its way. This water then piles up in a ring around the stone as it tries to move sideways, and then that "pileup" propagates away. Gravity furnishes the restoring force that strives to pull back down that raised ring of water. These waves are large and fast-moving, and are created by stones bigger than ~ a millimeter or two in diameter that strike the water at speed.

A capillary wave is formed when the stone deforms the surface layer of the water during its entry, which acts like a stretched and elastic membrane that pushes back against the falling stone. Surface tension furnishes the restoring force which strives to flatten back out the surface of the water. These waves are very small and slow moving, and are formed by stones less than ~ a millimeter or two in diameter that strike the water gently.