# Why does a liquid not rotate with the container?

This morning I made myself some coffee. There was cream on it as how I liked my coffee. I lifted my mug and rotated it so I can sip. Then I realized the cream on coffee wasn't rotating with the mug. It was in rest, the mug was rotating but the pattern of cream on my coffee was looking exactly the same. So the coffee had to be in rest or it was rotating the opposite direction I rotated my mug but why? Can anyone explain this phenomena or say the name of it?

• As an interesting and semi-relevant link here is an example of what can happen when the liquid in the container is incredibly viscous and DOES rotate when the container rotates... youtu.be/j2_dJY_mIys Apr 10, 2020 at 10:11
• It does rotate, it just takes awhile. Apr 10, 2020 at 11:20
• This is also the source of much frustration when drinking hot chocolate with marshmallows in it Apr 10, 2020 at 18:20

Suppose your cup was full of a lump of stuff that had no friction. Then as you rotate your cup there is no force between the cup and the stuff, and so it would not rotate. In the case of coffee - the friction is from the viscosity of the coffee. But, as your cup pulls some of the coffee around with it right at the edge, the rest of the coffee slips on that layer, rather like if you slip on a patch of coffee on the floor. Eventually, if you keep rotating the coffee, say on a turn table, the small friction will rotate all the coffee. Then, if you stop rotating the cup, the coffee will keep rotating from its inertia, until the small amount of friction has brought it to a halt.

• i.e. it does rotate...just not very quickly in response