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A falling stone takes 0.28 s to travel past a window 2.2 m..

  1. Sep 28, 2016 #1
    1. The problem statement, all variables and given/known data
    A falling stone takes 0.28 s to travel past a window 2.2 m tall.
    From what height above the top of the window did the stone fall?
    So from this problem I think I have these variables:
    vo = ? (or 0 because falling usually indicates 0 for an initial velocity?)
    yo = 0
    y = 2.2m
    t = 0.28s
    a = -9.8m/s^2

    2. Relevant equations
    Kinematic equations:

    y=yo+vo*t + 1/2*a*t^2
    v^2 = v^20 + 2a(y-y0)

    3. The attempt at a solution

    I originally tried to use the first equation above, and solve for vo if I assumed it was not equal to zero.
    So I isolated vo and got,
    vo = -2.2 + 1/2(-9.8m/s^2)(0.28s)^2 / -0.28s
    vo = 9.229m/s

    After which I tried to substitute that value in my second equation which also didn't give me the correct answer for height (which i think is the new yo)
    My textbook gives me the final answer but not the steps that it reached to get to it.

    I have included the final answer here in white ink (so highlight to read) In case anyone wants to check their work or work backwards: [COLOR=#black]2.1m [/COLOR]

    I feel like my process and identifying my variables given correctly is wrong and I'm at that point where I am confusing myself repeating this question.
    Guidance would greatly be appreciated,
    Thank you all!
    Newbie in Physics
  2. jcsd
  3. Sep 28, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    That's an OK method. But redo it more carefully.

    Since the stone is falling, that final value of y must be negative.
  4. Sep 28, 2016 #3
    If I change 2.2 to -ve I still get the same number -9.229, and I am still unsure what I do after?
  5. Sep 28, 2016 #4

    Doc Al

    User Avatar

    Staff: Mentor

    I get a different value for v0.

    Show what you've put into this equation:
    Don't isolate v0 yet, just show what you have for the values in that equation.
  6. Sep 28, 2016 #5
    Check your signs.
    If downward is negative then both V0 and g have negative signs, they both end up with negative signs in your final equation for V0.
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