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This is the problem

Spent an entire class period going over this and teacher just laughed at us and refused to guide us in the right direction. We

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    $\begingroup$ There should be a frictional force on A by B also. There may be other errors i don't know about it. $\endgroup$ – Guji2203 Jan 12 at 15:39
  • $\begingroup$ Here's a hint: Keep in mind Newton's third law. Identify all the forces on the two blocks that are applied externally and whose equal-and-opposite partners aren't identified (e.g. the earth is one of several such external objects). Then try to pair up all the remaining identified forces into equal-and-opposite pairs. $\endgroup$ – jwimberley Jan 12 at 15:40
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The problem is that the student forgot about Newton's third law: if a first object exerts a force on a second, the second must exert a force of equal magnitude and opposite direction on the first. Thus all force coming from interactions between A and B, which define our system, must come in pairs. This is where the student failed. For example, P is a force exerted on A by B. Thus, in the free body diagram of B there must be a force exerted by A. This is C. Then $(P,C)$ are a pair related by Newton's third law. There is however a force in one of the diagrams that is unpaired in the other. Can you see which one it is?

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