Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

People lifting themselves with simple fixed pulley

  1. Jan 19, 2016 #1
    I can't convince myself that the situation in this picture is possible in reality though I know the physics works.
    I feel that it won't work as the workers are not glued to the platform
    can any one find a demonstration to verify it .plz (I'v searched youtube as much as I could)
    people lifting themselves with simple fixed pulley (single person will also do)
    problems_MIT_rayyan_Physics801_Figures_Pulley_Platform.png
     
  2. jcsd
  3. Jan 19, 2016 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Why would you need glue if there is half the body weight as pressure between the person and the platform?
     
  4. Jan 19, 2016 #3
    There's no need to be glued on the platform. Imagine both persons hold their ropes and suddenly put their knees to the breast. The weight of the persons will lift the platform till both "sit" on it. Afterwards they could grab the rope at a point closer to the ceiling, by alternatley moving one hand above the other and keeping the tension in the rope with fixed hand while standing up (if they are physically strong enough).
     
  5. Jan 19, 2016 #4

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    What do you mean exactly by "it won't work"? What do you think will happen, if they are not glued to the platform (assuming they are heavier than the platform)?
     
  6. Jan 19, 2016 #5

    russ_watters

    User Avatar

    Staff: Mentor

    Since no numbers are provided, in order for it to work, we need to assume the weight of the people is greater than the weight of the platform.
     
  7. Jan 19, 2016 #6
    You don't even need a platform for this to work.
    You can pull yourself up with a rope thrown over a tree branch. You just make a little loop at one end so you put for foot through it and pull the other end of the rope.
    Look up "tree house elevator" to see more sophisticated contraptions, with multiple pulleys.
     
  8. Jan 19, 2016 #7

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Careful, we don't wanna the OP to hang her/himself. A small model with a basket and a weight would suffice.
     
  9. Jan 19, 2016 #8
    put some numbers into the question. Look at 1 person of mass 70kg (W = 686N) standing on a platform of mass 20kg (weight = 196N)....he pulls with a force of 400N on the rope.....can you calculate what is happening? the acceleration? can you draw a diagram showing the forces acting on the person and on the platform and on the rope??
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook