Sliding blocks and a string problem

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Block A, weighing 3w, slides down an inclined plane at a constant speed, while block B, weighing w, rests on top and is tethered to the wall. The problem involves determining the coefficient of kinetic friction between the blocks and the incline. The discussion highlights confusion regarding the free body diagram for block A, particularly concerning the forces acting on it, including normal forces and friction. There is a noted inconsistency in addressing the friction between blocks A and B despite recognizing the normal force between them. The resolution of these force interactions is crucial for solving the problem effectively.
Anatalbo
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Homework Statement


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Block A, with weight 3w, slides down an inclined plane S of slope angle 36.9 degrees at a constant speed while plank B, with weight w, rests on top of A and is attached by a cord to the wall. If the coefficient of kinetic friction is the same between A and B and between S and A, determine its value.

Homework Equations

The Attempt at a Solution


I have some trouble doing the free body diagram for A, I assume the forces acting on said block are: NB, wA, static friction between S and A, but I am not sure about the friction between A and B.
 
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Anatalbo said:
the forces acting on said block are: NB, wA, static friction between S and A, but I am not sure about the friction between A and B.
You mention static friction between S and A, but not the normal force there.
Why do you doubt friction between A and B, yet have listed "NB", which I take to be the normal force between A and B?
 
Thread 'Correct statement about size of wire to produce larger extension'

Thread 'Correct statement about size of wire to produce larger extension'

The answer is (B) but I don't really understand why. Based on formula of Young Modulus: $$x=\frac{FL}{AE}$$ The second wire made of the same material so it means they have same Young Modulus. Larger extension means larger value of ##x## so to get larger value of ##x## we can increase ##F## and ##L## and decrease ##A## I am not sure whether there is change in ##F## for first and second wire so I will just assume ##F## does not change. It leaves (B) and (C) as possible options so why is (C)...
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