What's Wrong with My Force Analysis in These FBDs?

[minimario]

Homework Statement

Homework Equations

F=ma

The Attempt at a Solution

A FBD on the bottom block gives tension in C is equal to Mg. Then, a FBD on the middle block should give T_b-T_c=0, so tension on B should also be equal to Mg. Finally, a FBD on the top block gives the tension on A is equal to Mg.

But this is not correct, what did I do wrong?

 

[phinds]

... so tension on B should also be equal to Mg.

Really? You might want to rethink that.

 

[minimario]

What are the forces acting on the middle block? Are they not only tension in B and tension in C?

 

[Chestermiller]

Draw a free body diagram of the middle block. Show the forces acting on that block. What are they?

Chet

 

[DTM]

For your 2nd FBD, on the middle block, you forgot to include the Weight of that block. Same for the FBD on the top block.

 

[phinds]

What are the forces acting on the middle block? Are they not only tension in B and tension in C?

Yes, but how does that relate to the answer to the you are looking for. Are you looking for the forces on a single block or the tension on the various strings? Suppose B was a rigid rod instead of a string. What would that do to your perception of the tension in string A ?

 

[minimario]

Ok, thanks. I got it now. The FBD on B should be T_b=T_c+Mg