Newton's laws of motion -- finding the velocity of a Block in a pulley system

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1. Jun 29, 2017

Vv anand

1. The problem statement, all variables and given/known data
Determine the speed with which block B rises in figures if the end of cord at A is pulled down with a speed of 2ms^-1

2. Relevant equations
Given Velocity downwards at A=2m/s

3. The attempt at a solution
Really stuck... Couldn't even start the question solving...I know all policies and therefore i am not demanding the question to be solved but just a hint on how to start will surely help

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2. Jun 29, 2017

haruspex

Create some variable names for the different unknown velocities. See what equations you can write to relate them.

3. Jun 29, 2017

Vv anand

Yes that i hv already done... Will start again.. Thx

4. Jun 29, 2017

Vv anand

Yea i started again but ended with nothing..I can show u my work if u want

5. Jun 29, 2017

Vv anand

This is what i did...i m stuck after this!

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6. Jun 29, 2017

Merlin3189

I don't know if it is allowed, but I used distances instead of velocities.
Just took a starting position, moved the block B 1 unit distance and calculated the distance each other bit moved if the string stayed tight.
That gave me a velocity ratio.

7. Jun 29, 2017

Staff: Mentor

If mass B moves up $\delta$, how much does the pulley that supports A move down?

8. Jun 29, 2017

Vv anand

Is it 3$\delta$

9. Jun 29, 2017

Staff: Mentor

No. Look at the diagram carefully.

10. Jun 29, 2017

Vv anand

Oh is it 4 $/delta$

11. Jun 29, 2017

Vv anand

Just because of the middle string attached to the mass coming from the upper left pulley

12. Jun 29, 2017

Staff: Mentor

No. Look carefully. The wire between pulley

13. Jun 29, 2017

Staff: Mentor

I get that the pulley supporting A moves down $\delta$ if mass B moves up $\delta$. This focuses on the wire that passes over pulley D.

14. Jun 29, 2017

Vv anand

Sir im coming to the solution that since pulley supporting B moves up the pulley supporting a moves down 3$/delta$ as i hv to ke

15. Jun 29, 2017

Vv anand

Yes sir...i got that the $/pulley$ is moving delta downwards

16. Jun 29, 2017

Vv anand

Wrote that upper comment by mistake...

17. Jun 29, 2017

Staff: Mentor

So if $v_B$ is the upward velocity of mass B, what is the downward velocity of the pulley that supports A?

18. Jun 29, 2017

Vv anand

Sir vb

19. Jun 29, 2017

Vv anand

But this brings me to the solution that vb=2/3.

20. Jun 29, 2017

Yes. So?