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I had an exercise like the image, where block A is pulled by a force F, there is that rope(tension) attached to the block B and the wall, and there is friction between A and B, and A and the ground, and the coefficent of friction is the same in both cases. The question asks us what is the intensity of the force required to start moving the blocks.

My doubt is: we know tension is only applied in block B, but are we supposed to apply it on block A as well? Because it is the only way I get a solution equal to the solutions of the book.
enter image description here

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  • $\begingroup$ The rope is attached to $B$. Therefore, there is no force from the rope on $A$. There must be another mistake in your work. $\endgroup$
    – Zo the Relativist
    Apr 22, 2016 at 22:21
  • $\begingroup$ Have you drawn a free body diagram on A and B separately, or do you feel that you have advanced beyond the point where you need to use free body diagrams? $\endgroup$
    – Chet Miller
    Apr 23, 2016 at 12:07

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You can use these free body diagrams:

enter image description here

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  • $\begingroup$ You might want to mention that contact forces act in equal and opposite fashion on contacting bodies. $\endgroup$
    – John Alexiou
    Apr 22, 2016 at 23:08
  • $\begingroup$ @ja72 Notice how he cleverly used labels like "F2" to indicate which forces have the same magnitude as each other? It's very subtle. $\endgroup$
    – Duncan Harris
    Apr 22, 2016 at 23:44
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    $\begingroup$ @ja72 I want to mention that free body diagram is the most important thing for solving dynamics problems. $\endgroup$
    – lucas
    Apr 23, 2016 at 0:02
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    $\begingroup$ I agree, a good sketch is 90% of the problem usually. $\endgroup$
    – John Alexiou
    Apr 23, 2016 at 1:04
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I know what you say, that's How I did initially, and it gave me a wrong solution. I will post the exercise to you solve by yourself. If I got the same system, without the rope, considering there is friction between the blocks, and A and the ground, is there friction between the blocks while I'm pulling block B but the system doesnt start moving?

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  • $\begingroup$ You can answer your question by plotting free body diagrams of blocks. $\endgroup$
    – lucas
    Apr 25, 2016 at 12:03

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