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Need help on another freefall problem.

  1. Oct 15, 2007 #1
    Need help on another freefall problem. :(

    1. The problem statement, all variables and given/known data
    A foul ball is hit approximately straight up into the air with a speed of 35 m/s.
    a) How high does it go (above the bat): Already figured this out, 62.5m. The initial velocity in this problem was 35 m/s.
    b) If it clears the backstop on the way up but gets caught and stuck in the backstop 4.00 meters above the bat on the way back down, how long is it in the air? (need help on this one)

    variables:
    t=?
    g=-9.8 m/s^2
    x=4 m
    x0=62.5m
    v0=0 m/s
    v=0 m/s

    2. Relevant equations
    x=x0+v0t+(1/2)at^2


    3. The attempt at a solution
    x=x0+v0t+(1/2)at^2
    4=62.5+(1/2)(-9.8)(t^2)
    -58.5=-4.9t^2
    t^2=11.93877551
    t=3.455253321

    Now, the correct answer to this problem is supposed to be 7.03s according to my teacher (he gave us the answers to check our work), but I obviously did something wrong and can't figure out what it is. I'm thinking maybe x isn't 4m, but I'm not sure. Any help is appreciated, thanks. :)
     
    Last edited: Oct 15, 2007
  2. jcsd
  3. Oct 15, 2007 #2
    First thing is first, there is no x in this problem. It may sound stupid..but do NOT use x as avariable when dealing with vertical displacement. USE y.

    Secondly..4m IS displacement...why do you have 62.5 plugged in for v_0 ?

    Casey
     
  4. Oct 15, 2007 #3
    62.5 isn't plugged into v0, I have 0 plugged into that. :confused: Did you mean y0?
     
  5. Oct 15, 2007 #4
    Oh. Sorry. But now that makes it doubly incorrect. Is v_0=0? Hint:no.

    What does the fact that the MAX HEIGHT that the ball travels is 62.5m have to do with this part of the problem? Hint: nothing.

    Casey

    Draw a Diagram and see what you can deduce from it and get back to me.

    What is v_0....y_0....y_f?....
     
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