# Sliding blocks problem

My first post here, so I apologize if this is duplicated elsewhere. It IS a "homework" problem, but it's public domain, a posted exam with answers...

Here is the diagram in question

And here is the FBD for each block. (question a is to find the tension in the rope and question b is to find the acceleration of the system)

I am getting confused when the situation has , for example, two blocks simply sliding over each other. In those cases, the force of friction on the top block, by Newton's laws, produces a "reactive" force in the opposite direction that makes the lower block move.

In this case, the force of friction OPPOSES the motion of both blocks. I would value some help with understanding this.

I.E. how would I compare this question with the one below... where there is friction between the block and toboggan, but the toboggan is on "ice" (no friction)

• I don't really get your confusion, it opposes the motion of both blocks. So what? Your body diagrams are fine so, all its left is a simple, though carefull application of Newtons Second Law. Pls explain your confusion further. – George Smyridis Mar 9 '16 at 20:12
• In the toboggan problem, friction opposes the motion of the block. So we have Fk acting RIGHT. The "equal and opposite" force on the toboggan will be Fk acting LEFT, which moves the toboggan. In the first question, Fk on the upper block acts RIGHT. Why is Fk also acting right for the lower block? And if it's acting right, where is the "equal and opposite" force on the lower block, how does it figure in the free body diagram? – Tom De Vries Mar 9 '16 at 21:25
• It's not acting right on the lower block. Again, it's acting left! Newton's 3rd law, always holds. Its one of the axioms that newtonean mechanics is based on. In your free body diagram the Fk is acting left, righ as it's supposed to. – George Smyridis Mar 10 '16 at 0:06