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?



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 same reason your body moves forwards when you’re standing in a bus and it brakes – inertia. All objects want to stay in their current state. What moves, wants to continue moving and what doesn’t move, wants to stay that way. Only when enough force is applied, their movement is changed.

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.

Wow, guys and gals thanks for responding so well and so quickly. You just answered a question that my high school teacher didn’t know how to answer

To add to all the good answer already made:

If you fill a glass with water and put it in the freezer and then turn it, you turn the ice together with the glass. If you wait a bit until a small layer of water has formed between the glass and ice and then turn the glass, you’ll find it will take less force and the ice won’t rotate with the glass.

This is because also turning the ice would use more power and nature always takes the path of least resistance. Only when it’s all frozen and bonded together the ice has to turn with the glass and so it will