Why do the ice and water in a cup stay in place if i twist the cup?

Question

You’d imagine since I’m moving the whole unit as one, the whole thing would move. But only the cup twists.

In: Physics

Answer 1

But you aren’t moving the whole unit as one. Your hand is touching the cup, not the water. So your hand moves the cup, and the cup moves the water. When you twist the cup, the cup tries to twists the water , but because the water is slippery and the smooth sides of the cup don’t provide anything for the water to grip, it only twists the water a tiny bit, so the water mostly stays in place.