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Homework Help: Having alot of trouble with projectile motion prob

  1. Sep 10, 2004 #1
    A football is kicked at ground level with a speed of 10.0 m/s at an angle of 39.0° to the horizontal. How much later does it hit the ground?

    i found the Voy to be 10sin(39), then i used Y=Yo + Vot + .5at^2 and i solved for t. I found t to be 1.97s

    but for some reason it is showing that i am wrong on my online hw, please help
  2. jcsd
  3. Sep 10, 2004 #2
    Your calculator is working in radians, but you're working in degrees.
  4. Sep 10, 2004 #3
    thanks man, i feel like an idiot, your awsome
  5. Sep 10, 2004 #4
    how did you set it up?

    Y=Yo + Vot + .5at^2
    0 = Yo + (10)t + .5(-9.8)t^2 <--a is -9.8 right? cause it's going down

    what is Yo?

    "i found the Voy to be 10sin(39)"

    isnt that the velocity? so you cant plug that in for Yo.

    how did you get your answer? i have the same type of problem
  6. Sep 11, 2004 #5


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    Homework Helper

    [tex] V_{o} = 10 m/s [/tex]
    [tex] \theta_{o} = 39^o [/tex]
    [tex] Y_{o} = 0 [/tex] The Football was at ground level
    [tex] T_{f} = ? [/tex] <- Flight time

    Remember we can work this problem with [tex] V_{o} [/tex] components
    [tex] V_{yo} = V_{o}\sin(\theta_{o}) [/tex]
    [tex] V_{xo} = V_{o}\cos(\theta_{o}) [/tex]

    We can use the equation you used.
    [tex] Y = 0 [/tex] when it hits the ground.
    [tex] 0= Y_{o} + V_{yo}t_{f} - \frac{1}{2}gt^2_{f} [/tex]
    [tex] 0= Y_{o} + V_{o}\sin(\theta_{o})t_{f} - \frac{1}{2}gt^2_{f} [/tex]
    [tex] 0 = V_{o}\sin(\theta_{o})t_{f} - \frac{1}{2}gt^2_{f} [/tex]

    Simply solve for Flight Time.
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