What elements of physics are involved in cup stacking?

[Emanuel Vellios]

I need to know how to relate this to physics. I've already said that gravity is used when you slide the cups down to stack them. What else can I say? Thanks.

 

[Drakkith]

I'd throw in some tidbits about friction since it's responsible for keeping the cups from sliding off of one another and for allowing you to pick them up in the first place.

 

[CWatters]

I've already said that gravity is used when you slide the cups down to stack them.

I suspect that serious competitors "push" the cups down faster than they would "fall" down due to gravity - so gravity only helps with the de-stacking.

Stacking involves moving your hands quickly, accelerating them and stopping them in the right place. Your hands have mass so perhaps mention Newton's laws?