1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: One dimensional motion problem

  1. Jun 11, 2009 #1
    1. The problem statement, all variables and given/known data
    Challenge Problem(2.96) from University Physics text book:
    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 1.00s in the air (their "hang time"). Treat the athlete as a particle and let Ymax 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/2 to the time it takes him to go from the floor to that height. You may ignore air resistance.

    2. Relevant equations
    constant acceleration equations: y=Y0 + V0t - 4.9t2....1

    3. The attempt at a solution
    Part 1(Time it takes him to go from floor to Ymax /2):
    ay= -g, origin at the floor, V0y=0, y=Ymax /2, y0=0

    so if i substitute these known quantities in the second equation, then for the velocity at the position Ymax /2 i am getting complex roots. Kindly inform me where i am going wrong.

  2. jcsd
  3. Jun 11, 2009 #2
    Try using a positive value for g. Since when you ultimately calculate time, using a negative value would result in a negative value under the square root.

    Edit: Strike out what I said.
    Last edited: Jun 11, 2009
  4. Jun 11, 2009 #3


    User Avatar
    Homework Helper

    No. if v0y=0 then how can he even get into the air? You can calculate what v0y is because you know what vymax is. What is vymax?
  5. Jun 11, 2009 #4
    Thanks Cyosis, I got it. since we know that Vymax is zero we can calculate the initial velocity of the part 2 which becomes the final velocity of part 1. am i right??
  6. Jun 11, 2009 #5


    User Avatar
    Homework Helper

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook