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Finding Take-Off Velocity

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  1. Feb 17, 2016 #1
    1. The problem statement, all variables and given/known data
    Kwaku Boateng is a former Canadian Olympic high jumper. If an average vertical ground reaction force of 1959.6 N acted on Kwaku's foot for 0.20 s during the upward phase of his jump, and Boateng's mass is 79.1 kg, what would his take-off velocity be?

    2. Relevant equations
    ΣFΔt=mΔv


    3. The attempt at a solution
    I tried it twice...

    Attempt 1:
    ΣFΔt=mΔv
    Δv=(ΣFΔt)/m
    Δv=(1959.6)(0.20)/79.1
    Δv=4.95m/s

    The online assignment says that this answer is incorrect.

    Attempt 2:
    ΣFΔt=mΔv
    Vf-Vi=(ΣFΔt)/m
    Vi=-[(ΣFΔt)/m]+Vf
    Vi=(1959.6)(0.20)/79.1
    Vi=-4.95m/s

    The online assignment says that this answer is incorrect.

    Should I be using another equation? Or have I made a simple mistake?

    Thank you in advance for your help!
     
  2. jcsd
  3. Feb 17, 2016 #2

    RUber

    User Avatar
    Homework Helper

    Do you need to include the force of gravity?
     
  4. Feb 17, 2016 #3
    Ahhhhh, yes that's it!

    ΣFΔt=mΔv
    Δv=(ΣFΔt)/m
    Δv=[1959.6-(79.1)(9.81)](0.20)/79.1
    Δv=2.99m/s

    The computer is telling me 2.99m/s is the correct answer, thank you for your help RUber!
     
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