If you throw an object into a lake why are the water ripples round?

Question

Why don't they take the shape of the section of the object that first hits the water? Or do they at first?

Answer 1

Every action has an equal and opposite reaction. The kinetic energy of the rock's motion gets imparted to the molecules it impacts which then transfer that energy to other molecules of water in a wave form. But, there is also surface tension to consider. The water, due to molecular charges and gravity wants to get back to level and the molecules move around to get to this settled state. They do this very quickly. So, yes, at the instant the rock hits the water, the water moves away from it in the shape of the rock, but then once the rock has vacated the area [i.e. dropped to the bottom] all those effects take over and the water molecules rush back in oscillating of each other and creating the familiar radiating waveform we're all used to.
However, if you throw the rock more along the surface as well as into it--not skipping, just thrown in at shallow angle, the ripple will not be round. Just look at a boat's wake as it moves into but more across the surface of the water.

Answer 2

the water is being displaced by this new object entering it. the ripples are a result of the displacement and disturbance at the point of entry. they dont take the shape of hte object because typically things will sink thru it because most objects are more dense than water. things that float also displace water, which is why the level of the water will rise in accordance to the amount of the object sitting below what was the present water line.

Answer 3

The ripples move out to all directions from the point of impact!
Water does not keep the shape of an object it takes the shape of an object.

Answer 4

This has to do with symmetry of the conditions and laws of physics. If the lake looks the same in the vicinity of the object's impact (no boats, waves or wind) if you rotate your view about about a vertical axis, and the rock itself is spherical and dropped vertically, the subsequent disturbance will have the same symmetry. That means round. I've heard this referred to (with tongue in cheek) as "the principle of minimum astonishment" .

As for your secondary question, they do take the shape of the object initially, but round out they travel further out because the speed of the wave is, more or less, constant. A brick twice as long as wide, for example, would have to have the long-side waves moving twice as fast to keep that shape.