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