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Homework Help: Finding Time For Projectile Motion

  1. Dec 15, 2011 #1
    1. The problem statement, all variables and given/known data

    A marble launcher shoots a marble with a launch velocity of 6.89 m/s @15.0 degrees above horizontal.

    -Find the marble's range.



    (I tried to do this but I need to be able to find the time for horizontal motion... how would I do that?)

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Dec 15, 2011 #2

    PeterO

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    Homework Helper

    The time is calculated by considering the vertical component of the motion, and treating it like any other vertical motion example.

    Or you could have a read of the

    http://en.wikipedia.org/wiki/Range_of_a_projectile

    then you only need a substitution - provided your projectile is landing at the same level it was launched from and not from/onto a hill/cliff/building.
     
  4. Dec 16, 2011 #3
    Because its 6.89m/s @ 15.0deg, you can use trigonometric functions to find the initial vertical and horizontal speed:

    vertical:
    sin(15.0) = o / 6.89

    horizontal:
    cos(15.0) = a / 6.89

    Since the horizontal velocity is constant as gravity does not affect it, the time the projectile is in the air can be found by:

    v = a*t

    where

    v = vertical
    a = acceleration due to gravity
    t = time of projectile

    Once you have the time of the projectile its simple.

    We know that distance = velocity * time

    -> d = v * t

    where d is the "range" we are trying to solve for
    where v is the HORIZONTAL velocity
    where t is the time of the projectile


    Once you have done this, post your work/solution so I can help you further. Nobody here is going to do you homework for you, we can only HELP you if you are trying.
     
  5. Dec 17, 2011 #4
    jack343, you are forgetting that the projectile goes up AND down so the time of flight is double what you calculate.
     
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