# Tension on the String

Tags:
1. Apr 11, 2015

### minimario

1. The problem statement, all variables and given/known data

2. Relevant equations
F=ma

3. 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?

2. Apr 11, 2015

### phinds

Really? You might want to rethink that.

3. Apr 11, 2015

### minimario

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

4. Apr 11, 2015

### Staff: Mentor

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

Chet

5. Apr 11, 2015

### 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.

6. Apr 11, 2015

### phinds

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 ?

7. Apr 11, 2015

### minimario

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