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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...

http://www.mun.ca/physics/undergraduates/finals/P1020F06.pdf

Here is the diagram in question

diagram 1

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)

Diagram 1

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) enter image description here

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    $\begingroup$ 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. $\endgroup$ Mar 9 '16 at 20:12
  • $\begingroup$ 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? $\endgroup$ Mar 9 '16 at 21:25
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    $\begingroup$ 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. $\endgroup$ Mar 10 '16 at 0:06
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The first thing first -The concept of friction Friction come in play when there is any tendency of relative motion between 2 surfaces and it is in opposite direction of relative motion (Note- I used word relative motion not simply motion). In simple word friction try to reduce relative motion. in 2 block problem and this problem the difference is T (tension). In both problem direction of relative motion is similar, friction is trying to do same thing i.e. pulling the other block with the one on whom force is applied so that relative motion can be reduced. But in this problem Tension forces block to move in other direction then supplied by friction.And but obvious T>Friction. so here T is increasing relative motion and friction as always trying to make them move together.

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You are right in all you have written. But a more fruitful sentence could be: Friction always tries to prevent sliding.

  • (Kinetic friction) If two boxes are sliding over one another, the friction on the top block will pull in the lower block's direction, and friction on the lower block will pull in the top block's direction. Friction tries to keep them together to reduce the sliding.

  • (Static friction) If two boxes on top of each other accelerate leftward (for example because a string pulls in the lower box), then friction in the top box pulls left to keep that block speeding up (prevention it from starting to slide - it is being "pulled along"), and friction in the lower box pulls right to make it stop accelerating and stay with the top one (again preventing sliding - it is being "held back").

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