If you turn a cup filled with water with a few pieces of ice, the ice does not appear to move. Why?


If you turn a cup filled with water with a few pieces of ice, the ice does not appear to move. Why?

In: Physics

Objects at rest remain at rest, unless acted on by an external force.

If the spinning glass isn’t touching the ice, the only way for it to transmit force to the ice is to first spin up the water. Since the glass is usually circular, the water is not forced to deform as the glass spins, and the only interaction is the slight friction between the water and the glass’ walls—essentially a simple [fluid bearing](https://en.wikipedia.org/wiki/Fluid_bearing). With very little friction between the glass and the ice, it takes a lot of glass-spinning to get the ice moving noticeably.

One feature that makes liquids liquid is that they can only achieve a certain amount of shear force. What this means is that one section of liquid can slide over the adjacent section pretty easily.

Imagine your glass of water being split up into thin concentric cylindrical sections. The section on the outside is touching the walls of the glass and so experiences some shear force from the rotation of those walls. It passes a bit of that force to the next layer in, and so on. Each layer experiences less force, but they all experience lots of resistance to movement because water is heavy and heavy things don’t like to change direction or speed; the water started stationary and wants to stay that way.

So quite rapidly as you move from the wall into the bulk of the liquid you find that it barely moves at all, and so the ice floating in the water doesn’t move either.

If you turn a cup filled with water, most the water in the cup doesn’t move at all either. Only the cup turns. Having the ice in there just lets you see that none of the cup contents are rotating.

When you turn the cup, only the water right against the walls moves along in their direction. The rest stays still because there’s nothing turning them. There’s very little friction between the cup’s walls and the liquid, and liquids don’t transmit shear (crossways) forces very much at all. If you spun the cup for a long time you could eventually get the contents turning as well.