[Physics] Friction between two blocks on top of each other

Question

I am a little bit confused to the concept of friction between two blocks on top of each other. there could be two situations:
the first situation is when all surfaces are frictionless including that one between the two blocks. if one force $F$ pulls the lower block, then I read somewhere that we should consider both blocks as one block and the upper block will remain at rest and so they have the same acceleration. but still, i don't get why it will remains at rest?
second situation:
if there is a friction between the two blocks, then applying an F force will make the upper block slide with it's own acceleration.
Is this right?
and regarding the force needed to make the upper block slide, is there any specifications?

Answer 1

Most of the confusion that arises in friction problems like this due to not drawing free body diagrams (FBD) showing all forces acting on the blocks.

The FBD below show the two scenarios you posed.

The left figure shows that the only horizontal force acting on the two blocks is the force $F$ applied to the lower block. There is no horizontal force acting on the upper block and therefore it does not move. There are no forces resisting motion of the lower block and therefore it moves with acceleration $a_Y=\frac {F}{M_Y}$ and slides underneath the stationary upper block and over the ground. The blocks will never move together.

The right figure shows equal and opposite friction forces acting on the upper and lower block. To make things simpler, we are assuming no friction between the lower block and ground.

You need to keep in mind that the friction force always opposes the applied force. Consequently, the friction force varies from zero, when the applied force is zero, to the maximum static friction force shown in the diagram, at which point the applied force causes impending sliding between the two blocks. Up until the maximum friction force is reached, the two blocks move together with the same acceleration ($a_X=a_Y$) with respect to the ground. So yes, the upper block will continue to have an acceleration equal to that of the lower block. However, once the applied force, $F$, reaches a level such that the static friction force reaches the maximum possible, slippage occurs between the blocks and the friction becomes sliding, or kinetic friction. The kinetic friction force is generally lower than the static friction. If the force $F$ is maintained, the acceleration of the lower block will be greater than the upper block.

Hope this helps.