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

Bungee Jumping-Tension+Air resistance with regards to max extension

  1. Sep 14, 2011 #1
    1. The problem statement, all variables and given/known data
    Bungee Jumping
    l=natural length of rope
    x=extension of rope
    y=total distance fallen
    m=mass,a=acceleration,v=velocity,g=acceleration due to gravity
    k=air resistance co-efficient

    Given data: The rope is stretched to twice its natural length by a mass of 75kg hanging at rest from its free end

    The bungee jumper jumps off a bridge, as he falls, he experiences quadratic drag (kv^2), when the rope begins to extend, he also experiences a deceleration due to the tension in the rope. Find the maximum extension of the rope (x) with regards to the mass of the jumper (m) and the natural length of the rope(l).

    Previous questions were without air resistance, which i worked out fine.

    2. Relevant equations

    F=ma (a=F/m)
    T=jx (Hookes Law, j not k to avoid confusion with drag)

    3. The attempt at a solution
    General thought pattern:
    Find the acceleration in terms of all the variables, from the acceleration, find the velocity through whichever means possible. Equal the velocity to 0, as this is when the extension will be at a maximum. Manipulate it into the form x=something.

    a=acceleration due to gravity-acceleration due to tension - drag
    a=g-(75g/lm)x -kv^2 ((75g/lm)x was found as the deceleration due to tension in one of the previous questions)

    So i now have a formula for a in terms of the extension and the velocity, the trouble comes when i attempt to substitute a for either (d/dx)0.5x^2 or v(dv/dx). As the formula contains both x and v, im always left with trying to do either (75g/lm)x dv or kv^2 dx. The solution my teacher came up with was to treat x or v respectively as a constant(which they're not), but i dont really think that is the appropriate way to go about it.
    I've also tried to use v^2 = u^2 +2as, but i then realised that only works for constant acceleration. I can calculate the velocity of the jumper when x=0, so im fine with tension or air resistance by themselves, it is when trying to put them both together where i become stuck.
    So im really just trying to find a way to get started on this question without running into a brick wall, the only thing i can currently think of at this point is equating x to v somehow, but i have no idea if that will work or not.
    Yes, this is for an assignment, i'm not asking for anyone to do it for me, im just looking for a place to start on this little piece of hell. If more info is needed, the assignment is here: http://www.qsa.qld.edu.au/downloads/senior/snr_maths_c_***_sample_2.pdf [Broken] (9th page) or just ask. Many thanks
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 14, 2011 #2
    link not working. please post the original question AS IT IS.
  4. Sep 14, 2011 #3
    Dynamics — Bungee jumping
    New Zealand is the home of bungee jumping. One of the
    major jumps is located on a bridge over the Shotover
    River near Queenstown.
    In this case, the bridge is 71 m above the river.
    Two types of jumps are available — wet and dry. In a dry
    jump, the person’s fall ends just above the water surface.
    In a wet jump the person is submerged to a depth of 1 m.
    Participants jump from the bridge, fastened to an elastic
    rope that is adjusted to halt their descent at an
    appropriate level.
    The rope is specially designed and its spring constant is
    known from specifications. For the purposes of the problem, we will assume that the rope is
    stretched to twice its normal length by a person of mass 75 kg hanging at rest from the free
    end. It is necessary to adjust the length of the rope in terms of the weight of the jumper.
    1. For a person of mass m kg, calculate the depth to which a person would fall if attached
    to a rope of the type described above, with length l metres. Treat the jumper as a
    particle so that the height of the person can be neglected. Discuss the assumptions
    made in this calculation.
    2. If you were the person jumping off the 71 m attraction, find the length of rope needed
    for a dry jump, where the descent is halted 1 m above the water.
    3. Now find the length of rope needed for a wet jump, where the descent would end 1 m
    below the surface of the water. Find the speed of entry to the water.
    4. In practice, the bungee rope is attached to the ankles of the jumper. Refine the
    previous model to allow for the height of the jumper and modify the earlier calculations.
    Is the difference significant?
    5. At present, the model does not include air resistance. Discuss the changes which
    would have to be made to the model to include air resistance, which is proportional to
    the velocity of the jumper. Discuss the difficulties involved with the mathematics of this
    6. Read the newspaper article entitled “Bungee jumping requires leap of faith” (available
    from the Bungee.com website, www.bungee.com/bzapp/press/lj.html). Use
    mathematics to support or refute the journalist’s comments.

    It is Q5 i am talking about
  5. Sep 14, 2011 #4
    I was skeptical as to what Q5 actually involved, but the teacher said i need to get a function x in terms of l and m
    Also, i know it says proportional, not quadratic drag, teacher said to change it
  6. Sep 14, 2011 #5
    thiras, in Q 5 they are saying that air resistance is proportional to the velocity of the jumper. why are you using quadratic terms in velocity for air resistance ?
  7. Sep 14, 2011 #6
    My teacher said that she thought that quadratic was easier to work with, and thats what she used in her workings. This might be a good time to point out that as of this morning she hasnt figured out how to do it yet, which is why i'm here asking you guys
  8. Sep 14, 2011 #7
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook