Suppose two rigid circular coins are lying on top of each other. A horizontal force on the top coin is applied. So, I think there are two possible scenarios: 1. Both move together with common acceleration (if friction between them is strong) 2. Only the top coin slips over the bottom (if friction between them is weak) I can understand the second case that when force on the top one is greater that the max value of static friction between them, then the top moves over the bottom with a net force. But I can't understand how the first can happen. If force on the top one is less than the max value of static friction, then friction between them applies an equal and opposite force to the top coin preventing it from moving. So, if force>max static friction, top slips over and if force<max static friction, net force on top becomes zero and no movement happens. Then, in what case exactly does they move together with common acceleration with friction acting as a connector? Also, what if unequal horizontal forces are applied on both the coins? In what case will they both move together and in what case one slips over another? Also, does friction act on the coin on which force is applied or does it act on the other one or does it act on both coins when one tends to move over other? I need a brief understanding of the concept, please.