This is the link to the figure I am using: http://web.ics.purdue.edu/~krousgri/m

Question

This is the link to the figure I am using:
http://web.ics.purdue.edu/~krousgri/me274/?p=2807&cpage=1#comment-10399

M_a = 50 kg
M_b = 75 kg
u_k = 0.2
u_s = 0.3
theta = 53.13 degrees

The system is being released in the configuration show. We want to find a) The acceleration of block A b) the acceleration of block B and c) Tension in the cable

I have drawn 2 FBDs (individual ones). I am stuck on the FBD of block A. I have my axes aligned with the incline, with 2T coming off the left side of the block in the -x direction, Normal force acting up on the block perpendicular to the incline, and of course M_a*g going straight down.

How do you decide which way the block A moves?? Then how do you decide whether it is slipping or not?

Explanation / Answer

It doesn't matter which way you draw the arrow on the free body diagram, if you get it wrong, there will be a negative in your answer when you do the force balance. As for deciding if it is slipping or not, when you find the forces in all directions, just check the frictional force (mu * normal) and see if it exceeds the static limitation.

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