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

  1. Jun 29, 2017 #1
    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. jcsd
  3. Jun 29, 2017 #2

    haruspex

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    Create some variable names for the different unknown velocities. See what equations you can write to relate them.
     
  4. Jun 29, 2017 #3
    Yes that i hv already done... Will start again.. Thx
     
  5. Jun 29, 2017 #4
    Yea i started again but ended with nothing..I can show u my work if u want
     
  6. Jun 29, 2017 #5
    This is what i did...i m stuck after this!
     

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  7. Jun 29, 2017 #6

    Merlin3189

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    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.
     
  8. Jun 29, 2017 #7
    If mass B moves up ##\delta##, how much does the pulley that supports A move down?
     
  9. Jun 29, 2017 #8
    Is it 3##\delta##
     
  10. Jun 29, 2017 #9
    No. Look at the diagram carefully.
     
  11. Jun 29, 2017 #10
    Oh is it 4 ##/delta##
     
  12. Jun 29, 2017 #11
    Just because of the middle string attached to the mass coming from the upper left pulley
     
  13. Jun 29, 2017 #12
    No. Look carefully. The wire between pulley
     
  14. Jun 29, 2017 #13
    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.
     
  15. Jun 29, 2017 #14
    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
     
  16. Jun 29, 2017 #15
    Yes sir...i got that the ##/pulley## is moving delta downwards
     
  17. Jun 29, 2017 #16
    Wrote that upper comment by mistake...
     
  18. Jun 29, 2017 #17
    So if ##v_B## is the upward velocity of mass B, what is the downward velocity of the pulley that supports A?
     
  19. Jun 29, 2017 #18
    Sir vb
     
  20. Jun 29, 2017 #19
    But this brings me to the solution that vb=2/3.
     
  21. Jun 29, 2017 #20
    Yes. So?
     
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