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Homework Help: Conceptual physics homework, easy, but need help understanding!

  1. Feb 23, 2013 #1
    1. The problem statement, all variables and given/known data
    Bronco Brown wants to put Ft =∆mv to
    the test and try bungee jumping. Bronco
    leaps from a high cliff and experiences
    free fall for 3 seconds. Then the bungee
    cord begins to stretch, reducing his speed
    to zero in 2 seconds. Fortunately, the cord
    stretches to its maximum length just short
    of the ground below.

    2. Relevant equations

    ∆mv at 3-s free fall is 3000 kg m/s
    Then the question asks: ∆mv during the 2-s interval of slowing

    3. The attempt at a solution

    To my knowledge it is at final velocity leaving it at 3000 kg*m/s
    Would it be 1500kg m/s or 3000kg m/s?
    I think 3000, but i'm not sure the equation to understand it.
    The equation I used gave me 1500 because of the slowing down.
    Please help me understand this better.
  2. jcsd
  3. Feb 23, 2013 #2


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    Gold Member

    Welcome to PF knnox!

    You are not communicating particularly well, I'm afraid. Nowhere in your problem statement does it actually state what you are supposed to work out. If the problem is to figure out "delta mv during the 2-second interval of slowing down", then that's fine, but I don't know how to check your work, because I don't see a mass stated for Bronco Brown anywhere.
  4. Feb 23, 2013 #3
    I apologize, it is 100kg, but looking back over this I now realize its just ask delta mv during the 2-s interval which is just 3000kg*m/s which is the same as impulse. Thank you for the response though and sorry for my lack of communication!
  5. Feb 23, 2013 #4


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    You can figure out his speed after free-falling for three seconds using v = v0 - gt, where v0 = 0, since he starts from rest at the beginning of the fall. I get 29.43 m/s for that, which when multiplied by 100 kg gives you a momentum of 2943 kg*m/s. (I guess you rounded up to 3000, but this introduces error). After that, you need to use the impulse-momentum theorem:

    FΔt = Δ(mv)​

    Here, Δ(mv) will be -2943 m/s, since he starts with 2943 kg*m/s, and ends with 0 kg*m/s, so the change in momentum is 0 - 2943.

    Now, given Δ(mv), you can solve for F in newtons, given that the impulse occurred over a time interval of Δt = 2 seconds.

    It sounds like you had it right (up to rounding error) in the first place.
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