when you slowly rotate the mug/jar/glass/etc, why does the water (or other drink) in it not rotate with it?
In: 15
It’s due to the boundary layer and depends on the viscosity of the liquid. The liquid which is touching the glass is called the boundary layer, you can consider it as a thin tube of water molecules which the rest of the liquid in the center is pushing out and sticking to the glass.
When the glass turns, this boundary layer moves with it because of the friction between the two materials.
Inside the boundary layer picture another, smaller tube of water molecules which interface with the boundary layer. When the boundary layer turns, this tube also turns, but because there is not a lot of friction between the two, not all of the energy is imparted and so it doesn’t move as quickly as the boundary layer.
This transfer of energy repeats until it reaches the centre of the liquid, or there is not enough energy to transfer to the next layer which, if all your doing is turning the glass, there won’t be.
If the liquid had a higher viscosity, then there would be a LOT more friction, which is why when turning a jar of honey, for example, all of the honey turns with the jar as though it were a solid, even though it is a liquid.
https://youtu.be/AfCyzIbpLN4 (4:18 for a visual explanation, but in your example it’s backwards)
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