1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hangmans Drop?

  1. Dec 16, 2007 #1
    OK here goes....

    I have a 20kg weight attached to a 10 meter long wire rope.
    The rope is fixed vertically to a cross beam.
    The rope has an elastic stretch limit of 47.5 kN/mm^2.
    If I lift the 20kg weight up 1 meter and then let it go what would the maximum force exerted by weight be on the wire rope?

    It's been a very very long time since I studied Newton so forgive me if I'm barking loudly up the wrong tree!

    I started of with Potential Energy = Mass X Gravity X Height but this didn't seem to cut the mustard so I then went for:

    Elastic Potential Energy = 1/2 x (k Spring Constant) x (L Spring Length)^2

    But I'm not sure if this is the correct equation to use....

    Impulse = (Mass x Velocity Initial) - (Mass x Velocity Final)

    Which again doesn't seem right but maybe I'm getting confused...

    Tension = Mass x Gravity x SQRT(2xHeight/Length Increase)

    So I have a selection of formulas that I think I should be using but I don't think I have the correct data to complete the formulas.

    So this is what I came up with but I don't really know if I'm going along the right lines or not.

    Velocity = sqrt 2 x9.81x1 = 4.429446918 m/s
    Potential Energy = 20x9.81x1 = 196.2 Joules
    Momentum = 20 x 4.429446918 = 88.58893836kg m /s
    Force = Change in Momentum / Time = 88.58893836 / 0.1 Second = 885.8893836

    So just how far wide of the mark am I?

    Attached Files:

  2. jcsd
  3. Dec 16, 2007 #2


    Staff: Mentor

    Use energy conservation, no need for momentum considerations here. The gravitational potential energy (h = 1m + x) at the top goes to elastic potential energy at the bottom (the rope stretches by x). Solve for x then put x into the elastic stretch formula to get the force.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Hangmans Drop?
  1. Oil drop (Replies: 1)

  2. Water drops! (Replies: 0)

  3. Voltage drop (Replies: 2)

  4. Dropping A Package (Replies: 2)