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Kinematics - free fall/projectile

  1. Sep 22, 2008 #1
    1. The problem statement, all variables and given/known data
    The problem is that basically, if an object is thrown up with a certain initial velocity (v), and an object at height h directly above the first object falls (intial velocity=0), write an equation for the position they collide. (position denoted as x)

    2. Relevant equations

    3. The attempt at a solution

    ok I tried this question so many times (actually about 3 times). This is the equations I used

    object 1 (falling down)
    -x= -g/2t^2

    object 2
    h-x= vt - g/2t^2

    I isolated them for x, but the answer is wrong. Can anyone tell me what is wrong with my 2 equations?

    edited to add: this is what the 2 equations solved to:
    Last edited: Sep 22, 2008
  2. jcsd
  3. Sep 23, 2008 #2


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    Hi rhodium! :smile:

    If v is measured up, then s must be measured up also,

    so the falling one is x= -gt^2/2 (or x = h - gt^2/2 , depending where you're starting from :wink:)

    and the other one is … ? :smile:
  4. Sep 23, 2008 #3

    i am still confused. i put - on the s because the displacement is negative. but sicne the initial position is h, then it would make sense to write it that way. thanks!

    ok, so, for equation 2 (object going up), would it be like this:

    y=vt-g/2t^2 where x=y?

    i hope i got it right.

    edited to add:

    I isolated for x.
    Last edited: Sep 23, 2008
  5. Sep 24, 2008 #4


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    Hi rhodium! :smile:

    Yes, if x = h - gt2/2, then y = vt - gt2/2. :smile:

    No … put x = y, and solve for t … then use that t to get x. :smile:
  6. Sep 24, 2008 #5
    all right,

    thank you for teaching me. i was really stuck before.
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