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Homework Help: Time at which a projectile reaches a certain height

  1. Jan 30, 2009 #1
    1. The problem statement, all variables and given/known data
    Hey, I think I understand this question but there's one or two parts that are really confusing me, the question is:
    A major leaguer hits a baseball so that it leaves the bat at a speed of 31.0 m/s and at an angle of 36.1 degrees above the horizontal. You can ignore air resistance.
    At what two times is the baseball at a height of 10.0 m above the point at which it left the bat?

    2. Relevant equations
    I realize that the equation I should be using is:
    y=(v0sin)*t- 1/2*g*t2

    3. The attempt at a solution
    so far I've tried various things but I fee like I'm missing something cause I cant figure out how to get t, is there some variabe or function for t that I can substitute in order to get the equation?
  2. jcsd
  3. Jan 30, 2009 #2


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    Homework Helper

    You could do it by trial and error - put in various times and see when y = 10.
    Graph the whole thing on a calculator and trace until you find points with y = 10.

    When you replace y with 10, you will have a quadratic equation which can be solved in various ways - notably the quadratic formula.
  4. Jan 30, 2009 #3
    that's nice and all, but it isn't going to help me on the test
  5. Jan 30, 2009 #4
    find out it's maximum height and the time it reaches it. from there you can add or subract time to find when it's 10m high
  6. Jan 30, 2009 #5
    Im pretty sure what delphi said WILL help you on the test and is the best way to go about the problem.

    you were given a y-function and you know that you are looking for a height of 10m. You know the angle, Vo and g. As delphi said, that gives you a quadratic. You solve the quadratic formula for values of t when the ball is 10 meters high. You'll get two values for that...
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