Free body diagram

  • Thread starter 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?

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The Attempt at a Solution

 

Answers and Replies

  • #2
Doc Al
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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.
 
  • #3
SteamKing
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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.
 
  • #4
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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?
 
  • #5
Doc Al
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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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