What is the Correct Free Body Diagram for a Pulley System?

Join the discussion
Registration is free. Start your own thread to ask a follow-up.
6 replies · 12K views
Goofball Randy
Messages
21
Reaction score
0

Homework Statement



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

YF-05-41.jpg

Homework Equations



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

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:
Physics news on Phys.org
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?
 
Nathanael said:
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?

For part A, it's of the pulley system. B and C ask you to draw for one of the weights.
 
Did you get B and C right? For A you described an FBD for the weight, not the pulley.
 
Nathanael said:
Did you get B and C right? For A you described an FBD for the weight, not the pulley.

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...
 
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?
 
Nathanael said:
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?

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...