Free Body Diagram: Explaining 3 Cord Tensions for Block C

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Homework Help Overview

The discussion revolves around the free body diagram (FBD) of Block C in a system involving cord tensions and pulleys. Participants are exploring the reasoning behind the inclusion of three cord tensions in the FBD and the implications of considering the bottom pulley as part of the system.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning whether Block C should be considered alone or as part of a larger system that includes the bottom pulley. Some suggest that including the pulley simplifies the analysis, while others argue that it complicates the FBD by introducing additional tensions.

Discussion Status

The discussion is active, with participants offering different perspectives on how to approach the FBD of Block C. There is recognition that both methods of considering the system are valid, and guidance is provided on the implications of each approach.

Contextual Notes

There is a mention of the need for additional diagrams if Block C is considered separately from the bottom pulley, indicating that the setup may involve complexities that require careful consideration of the system boundaries.

princejan7
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Homework Statement



http://postimg.org/image/5h0hvdi4l/
Can someone explain why the FBD of Block C would involve three cord tensions?

Homework Equations





The Attempt at a Solution

 
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princejan7 said:
Can someone explain why the FBD of Block C would involve three cord tensions?
Whoever is claiming that is probably considering the bottom pulley, which is connected to block C, as part of the block C system. Perfectly OK.

But you certainly do not have to! If you consider block C separately, you'll also need a diagram for that bottom pulley.
 
Can someone explain why the FBD of Block C would involve three cord tensions?

It can't. There are only two cords directly connected to block C.

Now, if you were to make a horizontal cut between the two smaller sheaves, you would have the tension in three cords to consider.
 
Doc Al said:
Whoever is claiming that is probably considering the bottom pulley, which is connected to block C, as part of the block C system. Perfectly OK.

But you certainly do not have to! If you consider block C separately, you'll also need a diagram for that bottom pulley.
http://postimg.org/image/oxb74l6t1/

so here they've considered the block + the bottom pulley as the system?
is there any reason for why they did that instead of just considering Block C as the system?
 
princejan7 said:
http://postimg.org/image/oxb74l6t1/

so here they've considered the block + the bottom pulley as the system?
Yes, for the purpose of that FBD they consider 'block + bottom pulley' as a single unit.

is there any reason for why they did that instead of just considering Block C as the system?
Just saves a bit of time. If you just considered Block C alone, you'd have to do another FBD for the bottom pulley to figure out the tension in the cable connecting it to the block. No big deal; either way is fine.
 

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