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Homework Help: Projectile Motion of a football please help!

  1. Oct 18, 2004 #1
    Ugh! I have been working on this lab for hours now and I can not seem to make sense of my results. We went to the park and kicked a football, recorded its hang-time, and measured the distance it traveled. This is the information I have:

    Time Change in x (m)
    3.69 sec. 42.06

    This is everything I know:

    Change in x = 42.06 m Change in y = 0 m
    a = 0 m/s a = -9.8 m/s^2(gravity)
    Vx = ? Vy = ?
    Time = 3.69 s Time: 3.69 s.

    I need to find the initial(overall) velocity of the football as it is kicked. I need to find what this would be if it were kicked at a 30 degree angle, a 45 degree angle, and a 60 degree angle.

    So far i have found that Vx = 11.4 because:
    Change in X = VxT + 1/2aT^2
    42.06 = Vx(3.69) + 1/2(0)(3.69^2)
    Vx = 42.06/3.69
    Vx = 11.4

    I found Vy with the same formula and found it to be 18.081

    I made a right triangle and gave it a 30 degree angle.


    Sin30= 18.081/V (v being the overal velocity)
    You get 36.162

    This is where the problem hits. Technically, by taking the cos30 of 11.4, I should get the same number. But instead:
    cos30=11.4/V
    V = 11.4/cos30
    V = 13.16

    Please help me figure out what i'm doing wrong. I did examples in the book and everything worked fine with those numbers, I don't understand why they are not working with these. I know that the time and measurements are right bc it was given to us on the board.

    Please help!
     
  2. jcsd
  3. Oct 18, 2004 #2
     
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