You’d imagine since I’m moving the whole unit as one, the whole thing would move. But only the cup twists.
In: Physics
I am no expert. But you can imagine water as separate particles which attract each other with a certain amount of energy. When you twist your cup the outer most water molecules are getting accelerated, but the attraction between the molecules is too low to accelerate the other molecules in the inner neighborhood.
On the other hand sand. Sand has a high friction value. The sand in the cup seems to rotate instantly. This is due to the high friction value. The inner neighbors of the sand particles get accelerated almost instantly. Maybe if you would use dry and smooth sand and twist your cup very fast, then the effect could look very similar to your cup-water-twist-experiment.
It’s the law of inertia. Things at rest want to stay at rest and things in motion want to stay in motion. Since water provides very little friction the ice continues to stay at rest when you turn the cup. If you turn the cup faster or have a thicker thicker liquid you would be able to provide enough force to get the ice moving with the cup.
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.
The water isn’t attached to the cup. The only force between the cup and the water is friction between the surface of the cup and the water molecules directly next to it. When you spin the cup, this force will push on the water a little bit, but not very much. If you keep spinning the cup, though, this friction will eventually cause the water to catch up with the cup and rotate along with it. But if you then stopped spinning the cup, the water would actually keep spinning and take a while to come to a stop, for the same reason.
If instead of liquid water, you had a block of ice frozen to the inside of the cup, then the molecules in the ice are all connected to each other and to the cup by rigid bonds. Thus, any force that you apply to the cup is also applied to the ice. So in that case you really can think of it as a single “unit”.
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