Why is it that if I put an ice cube in a glass of water and turn the glass, the ice cube does not rotate with it?
In: Physics
The ice cube has inertia and doesn’t want to spin
When you spin a glass full of water with an ice cube in the middle, you are *only* spinning the glass.
Friction between the glass and the thin layer of water right up against it will cause that layer to spin at the same speed as the glass, but that water then pushes loosely on the water on the inside which pushes loosely on water inside of it and so on.
The end result is when you quickly spin the glass, very little of that rotational force actually gets transferred to the ice cube in the middle, and its inertia is relatively high (compared to the water around it) so it stays pretty much stationary
If you were to put the glass on a motorized table and start it spinning you would see the ice cube stay stationary for a bit then pickup speed until it, and all the water in the glass, is spinning at the same rate as the spinning table, but by hand its hard to maintain a spin for the multiple seconds required.
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