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Knematics finding velocity

  1. Nov 1, 2008 #1
    1. The problem statement, all variables and given/known data

    A person throws a rock horizontally from the roof of a 50m building. The rock lands 70 m from the base of the building.
    How long was the rock in the air in s?

    I got 3.194. The online home work thingy (webassign) said I was right.

    With what velocity was the rock thrown in m/s?


    2. Relevant equations
    I used the Xf equation and solved for time. Dont I just plug in the time i got in the last equation and actualy solve it? Well i tryed that but I just keep getting it wrong.


    3. The attempt at a solution
    70=-4.9(3.194)^2 + Vo(3.194) Is this right?
     
  2. jcsd
  3. Nov 1, 2008 #2

    djeitnstine

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    The way you need to set it up is imagine the rock is simply dropping because the x velocity doesn't matter. So s = vt + 1/2 at^2 s = 50m v = 0 a = 9.8m/s^2
     
  4. Nov 1, 2008 #3

    djeitnstine

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    50 = 0t + 1/2 9.8 t^2
     
  5. Nov 1, 2008 #4
    what does s represent in this?
     
  6. Nov 1, 2008 #5
    I think i did my algebra wrong because it said it was wrong.

    (49.88+50)/3.194= 31.305
     
  7. Nov 1, 2008 #6

    djeitnstine

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    sorry s is displacement
     
  8. Nov 1, 2008 #7

    djeitnstine

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    50 = 0t + 1/2 9.8 t^2 this is the equation, the height 50 is equal to zero plus 1/2 9.8*t^2 (the rock is modeled as only dropping from 50 meters above the ground the x velocity has nothing to do with it)
     
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