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Homework Help: A Stone Thrown Downward

  1. Jan 26, 2013 #1
    1. The problem statement, all variables and given/known data
    A person throws a stone vertically downward with an initial speed of 16.0m/s from the roof of a building 58.2m above the ground.
    What velocity does the stone have at impact?

    2. Relevant equations

    initial x= 58.2m initial vox=16.0m/s ax= 9.81m/s^2 xfinal=0 vx=?

    2a(x-xo)=vx^2-vox^2 to find t= 3.88

    vx=vox+axt plug in a number including t=3.88 vx= 54.1

    3. The attempt at a solution
    2 attempts with only 2 attempts left. Please help
  2. jcsd
  3. Jan 26, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    Simplest way is use nergy conservation: initial kinetic energy + loss of potential energy = final kinetic energy.

    Other way: v = v0+ gt
    h = ∫v(t)dt = v0t + gt2/2 & solve for t = time of fall
    h = height of building
    Then stick t into the v equation above.
  4. Jan 26, 2013 #3
    I am still confused on how I would plug the numbers because the instructor have me equations with v and x variables. However, are my numbers correct for time=3.88 s and my vx = 54.1 m/s which I'm sure isn't correct. I'm solving for vx
  5. Jan 26, 2013 #4


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    Staff: Mentor

    t doesn't appear in the equation, so how do you obtain t from it :confused: (Also, that doesn't look like a correct value for t for this situation).

    However, you can use that equation to find the final velocity, vx. Rearrange the equation to solve for vx.
  6. Jan 27, 2013 #5
    Ok I got it! LIke gneill said, I could rearrange the equation and solve for vx. Thank you all.
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