# Homework Help: Free Body Diragram of a pulley

1. Feb 21, 2015

### Goofball Randy

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

In the figure (Figure 1) each of the suspended blocks has weight w. The pulleys are frictionless and the ropes have negligible weight.

2. Relevant equations

w = mg
Forces cancel out, so Tension = weight (since they are suspended)

3. The attempt at a solution

For a (pulley) ), I tried a free-body diagram with mg pointing down, and tension pointing up, with equal magnitude. This was marked wrong, but I don't see any other forces acting on the weight.

For b (either weight) ), I did the same thing as I did in a, because, after all, isn't it just mg and tension that act on both weights?

For c (either weight) ), same as a and b.

Last edited: Feb 21, 2015
2. Feb 21, 2015

### Nathanael

Exactly what does the problem ask? To draw a free body diagram of the pulley? If so then why did you draw FBDs of the weights?

3. Feb 21, 2015

### Goofball Randy

For part A, it's of the pulley system. B and C ask you to draw for one of the weights.

4. Feb 21, 2015

### Nathanael

Did you get B and C right? For A you described an FBD for the weight, not the pulley.

5. Feb 21, 2015

### Goofball Randy

No, B and C are wrong as well :( That's why I don't understand, I would have thought it would just be tension and mg...

6. Feb 21, 2015

### Nathanael

Your teacher didn't explain why you got it wrong? I would ask him/her why you got B and C wrong. Do you know why you get A wrong?

Do you have the exact problem statement?

7. Feb 21, 2015

### Goofball Randy

That is the exact problem statement. :(
In the figure (Figure 1) each of the suspended blocks has weight w. The pulleys are frictionless and the ropes have negligible weight.

In the case (a), draw the free-body diagram.
In the case (b), draw the free-body diagram of one of the blocks.
In the case (c), draw the free-body diagram of one of the blocks.

Word for word.

And this is one of those online assignments. I can't get any teacher feedback until the week begins.

The problem itself doesn't say whether it's wanting the pulley or the weight. But given that I've tried tension up, mg down, and that's incorrect, and I believe that to be the correct FBD for a suspended weight, I'm led to believe that it's asking for the pulley system...

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