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