# When rotating a liquid-filled jar what determines how much force goes to spinning the specimen floating inside?

Another way of stating it: When you have leaves in a (cylindrical) tea cup, if you rotate the cup (eg clockwise), the leaves largely stay in place relative to the table, but spin clockwise a little. What factors go into determining how much force (spin? torque?) transfers to the tea leaves (or to a neutrally buoyant object inside the cup)

I'm looking for highly simplified mechanics that can be translated into real-time rigid-body 2D physics for a video game (eg circles containing neutrally-buoyant specimens of various shapes). Having the circles bounce around like billiard balls is easy enough but I have no idea how to make them appear to be specimen-filled.

Any help is appreciated as I have little clue what to search for. I'm trying to avoid complex/super-realistic models/simulations (eg one that would contain many particles), but I'd be interested in the physics of it all, all the same.

• Too short for an answer: In case of the teacup maybe see it as solid body rotation with some friction between outer shell (cup), and inner body (tea). The tea leaves are just part of the solid inner body in that case. – Bernhard May 6 '14 at 19:09
• I see two cases. 1) spinning the cylindrical container about its central axis. In that case viscosity is the only effect. 2) If the container has some air above the liquid, and the container is not being rotated but is being cycled around an off-center axis (like swishing the coffee in a cup without turning the cup), that is different, and causes much more powerful rotation of the liquid. – Mike Dunlavey May 6 '14 at 19:27