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Homework Help: Projectile motion question (baseball problem)

  1. Dec 29, 2008 #1
    1. The problem statement, all variables and given/known data

    The center of a baseball of diameter 8.20 cm is 1.320 m vertically above the plate when it is hit. The blast sends it off at an angle of 30.5° above the horizontal with an unknown initial speed. The outfield fence is 94.0 m away and 11.20 m tall: the ball just clears it. Ignoring aerodynamic effects, what was the initial speed of the baseball?

    2. Relevant equations

    -d=vt for the x direction
    -any free fall equation for the y direction

    3. The attempt at a solution

    dx= 94.0m dy= 9.88m (subtracted the height in which the ball was hit at)
    Vx= vcos(30.5 Viy=?
    t= 94.0/vcos(30.5 a= -9.8m.s^2

    d=vit+1/2at^2
    9.88=vi(94.0/vcos(30.5)-4.9(94.0/vcos(30.5)^2
    v^2(-99.22)= -58323.769
    v^2= 5884.13
    v=76.7m/s

    where am i going wrong in this problem? help will be greatly appreciated, thanks!
     
  2. jcsd
  3. Dec 29, 2008 #2

    PhanthomJay

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    You have Vx=vcostheta; how about Viy=vsintheta? Looks like you have missed that.
     
  4. Dec 29, 2008 #3
    thanks for the reply. do i add the diameter of the ball to when it clears the fence too? i'm pretty bad with word problems.
     
  5. Dec 29, 2008 #4

    PhanthomJay

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    i guess to be accurate to 2 places after the decimal point, you'd have to consider the ball diameter, that is, add half its diameter to the y height (y = 9.88 + 0.04 = 9.92) for what it may be worth.
     
  6. Dec 29, 2008 #5
    i'm still getting this problem wrong so my equation in the y direction would be

    9.92=vsin(30.5(94/vcos(30.5)-4.9(94/vcos(30.5)^2

    i come up with the answer of 3.88 m/s but my problem set says it's wrong

    should i be doing something different?
     
  7. Dec 29, 2008 #6

    PhanthomJay

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    Looks like the correct approach; I'll check your numbers in a bit...I've got to pick up a take out order for the family..
     
  8. Dec 29, 2008 #7

    PhanthomJay

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    Looks like a math error..I get V = 35.99m/s. Check your parentheses..check calculator set to degrees and not radians..
     
  9. Dec 29, 2008 #8
    thank you very much for the help!
     
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