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Homework Help: Ball projectile physics

  1. Mar 7, 2005 #1
    1)A ball is thrown upward with an initial velocity of v = 20 m/s. How long is the ball in the air? What is the greatest height of the ball? When is the ball 15 m
    above the ground?

    1) I have figured out

    t total = 4.08s
    greatest height = 20.4m

    For the last bit (Finding the time when x=15m), I am having trouble. I know the answer but cannot work it through.

    Do I use s = ut+1/2at^2 for it?

    If I do, it wont work out because the 20.4 m has to come into it, doesnt it?

    Please help!
  2. jcsd
  3. Mar 7, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    The equation giving height (let's label it through 'y') as function of time is
    [tex] y(t)=v_{0}t-\frac{1}{2}gt^{2} [/tex]

    Now set "y" to 15m and solve for "t"...

  4. Mar 7, 2005 #3
    Cheers mate
  5. Mar 9, 2005 #4
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