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Pulley problem!

  • Thread starter gunni91
  • Start date
  • #1
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Homework Statement



Need some serious help with this one :O

A box with a weight WA hangs in pulleys ,(see figure 1). Find the force to pull A up with constant speed and calculate the work which the tractive force F produces ( with the movement of s) and also the work and tractive force T1 produce .
Pulleys and strings are frictionless and both without mass!.

pulley.jpg


Homework Equations






The Attempt at a Solution

 

Answers and Replies

  • #2
ehild
Homework Helper
15,406
1,809
What have you tried so far?

ehild
 
  • #3

Homework Statement



Need some serious help with this one :O

A box with a weight WA hangs in pulleys ,(see figure 1). Find the force to pull A up with constant speed and calculate the work which the tractive force F produces ( with the movement of s) and also the work and tractive force T1 produce .
Pulleys and strings are frictionless and both without mass!.

pulley.jpg


Homework Equations






The Attempt at a Solution

taikng tensions ,T1=4F
and taking constrained motion,
acc. of pulling end=4 times acc. of wA
differentiating that we get s=4s1
therefore work done by F=Fs
work done by T1=T1s1=4F x s/4=Fs

correct me if i m wrong,i just tried.
 
  • #4
ehild
Homework Helper
15,406
1,809
The result is correct. Balance of forces gives that WA=T1=4F. The question is the force needed to move the load with constant speed, so write out that F=WA/4.
The mass moves with constant speed so there is no acceleration. The constraints apply for the displacements and changes of length of the pieces of the ropes, but not for the non-existent accelerations. You can say that moving the end of the left rope by s will cause the other two pieces shorten by s/2 so the middle pulley raises by s/2. In the same way, raising the end of the right rope by s/2 lifts the right pulley by s/4.

ehild
 
  • #5
thank you echild.
 
  • #6
sorry didn't see the name
properly,very sorry.thanks ehild!
 
  • #7
ehild
Homework Helper
15,406
1,809
sorry didn't see the name
properly,very sorry.thanks ehild!
You are welcome :smile:

ehild
 

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