[Physics] Why water in the sink follow a curved path

angular momentumfluid dynamicspressurevortexwater

When you fill the sink with water and then allow the water to be drained, the water forms a vortex.. And then it starts to follow a curved path downwards by effects of gravity.

Why this phenomena occurs while rain follows a straight line path (in perfect conditions) towards the ground.

I would guess that when the water molecules closest to the drain hole go through first, they create a temporary void causing other water molecules nearby (above and on the sides) to rush in and take their place each having an equal chance to fill that void (since pressure is equal in all directions at a certain point in liquids). So I was thinking more of cone like figure with water collapsing in, equally from each direction. Why the circular path??

Best Answer

The difference between rain and water in the sink is that rain is simply falling, while water in the sink is being drawn into a center from a distance away, and the water in the sink is not perfectly still. It is rotating, if only a little bit. As it is drawn to the center, the rotation becomes more rapid.

The principle is Conservation of Angular Momentum.

Here, Ms. Kawaguti speeds up her rotation by pulling in her arms:

enter image description here