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Homework Help: Vertical jump

  1. Jan 29, 2013 #1
    1. The problem statement, all variables and given/known data
    In the vertical jump, an athlete starts from a crouch and jumps upward to reach as high as possible. Even the best athletes spend little more than 1.00 s in the air (their "hang time"). Treat the athlete as a particle and let y_max be his maximum height above the floor.

    To explain why he seems to hang in the air, calculate the ratio of the time he is above y_ max/2 to the tme it takes him to go from the floor to that height. You may ignore air resistance.

    2. Relevant equations


    3. The attempt at a solution

    From floor to ymax


    From ymax/2 to ymax

    ymax/2 = vo*t - 1/2*g*t^(2)

    Not sure what to do after here.
  2. jcsd
  3. Jan 29, 2013 #2


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    Science Advisor

    You mean ymax= (g/2)t^2. You don't say how you are setting up your coordianate system, but assuming positive is "up", ymax is positive.

    Please don't use "t" for both times! t in your first equation is the time necessary to get to the highest point. Let's use "s" to mean the time to the half that. Now you have, of course, [tex]ymax/2= (g/2)t^2= v_os- (1/2)gs^2[/tex]. That is a quadratic, [tex](g/2)t^2- v_os+ (g/2)s^2. Solve that for quadratic for s in terms of t, and form the ratio.

  4. Jan 29, 2013 #3


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    2017 Award

    Staff: Mentor

    As you have two different times, I would use t and t' to separate them. In addition, try to express vo as function of t or ymax.
    You can then use your equations to find an equation with t and t' and no other variables.
  5. Jan 29, 2013 #4
    Thanks guys!
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