# Explanation about friction force directions of two moving bodies in contact

can someone explain me why in this scenario (picture below) we have a friction force Ftr12 in the same direction as movement (left part of the picture, case 1 ) and how these directions of friction force match the ones on the right case ...

One remark: I understand the case on the right, but not completely the on the left, so I need clarification with it ...

Ftr12 means the force which body 1 acts on body 2, correct ? And for Ftr21 means the opposite ?

Here is the picture

P.S. My theory is that when the body 2 starts moving, body 1 goes along with it and is affected from resistance force of air which wants to move it to the left...So Ftr12 is the reaction to that force, that's why it is intent to the direction of movement.

Also if we let these bodies move indefinitely, would the body 1 slide off from body 2, given the V1>V2 ?

Also, additional question, what would be the case with direction of friction forces if V2>V1 and would the body 1 slide off backwards from body 2 ?

Generally, yes, the numbers $A_{mn}$ mean "The $A$ with which body $m$ acts on body $n$", for whatever the $A$ is.