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Can of paint falling down

  1. Apr 11, 2007 #1

    ssb

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

    A painter is standing on scaffolding that is raised at constant speed. As he travels upward, he accidentally nudges a paint can off the scaffolding and it falls a distance [H1] to the ground. You are watching, and measure with your stopwatch that it takes a time of [T] for the can to reach the ground. Ignore air resistance. What is the speed of the can just before it hits the ground (final velocity)?

    2. Relevant equations

    y = Yi + ViyT + 1/2 (AyT^2)

    Vy = Viy + AyT

    Vy^2 = Viy^2 + 2A(Y-Yi)

    3. The attempt at a solution

    Ive done this problem with each of the given equations and none seem to work. I was given sample values for H1 = 15 meters and T = 3.25
    The answer is suppose to be 20.5 m/s but ive plugged in these numbers a hundred times and gotten several different answers.
    Can someone please tell me what equation to use and how to arrive at 20.5 m/s for final velocity? Thanks
     
  2. jcsd
  3. Apr 11, 2007 #2

    Dick

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    You need to use your first equation for y to find Viy. Then use the second equation to find Vy. Keep all of the signs straight.
     
  4. Apr 11, 2007 #3

    Päällikkö

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    The scaffolding is going up, thus Viy is nonzero. You should be able to figure Viy as you're given T (and H1).

    EDIT: I'm too slow.
     
  5. Apr 11, 2007 #4

    ssb

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    and

    Thank you so much for this insight. I am going to try it right now

    First equation
    -15 = 0 + Viy(3.25) +(1/2)(-9.8)(3.25)^2

    Viy = 5881/520

    Second equation
    (5881/520) + (-9.8)(3.25) = Vy = -20.5

    Whohoo! the number is correct but it looks like I messed up somewhere with the signs because it should be positive. Hrm.... any ideas?
     
  6. Apr 11, 2007 #5

    Dick

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    The paint can IS going down when it hits, right? Wouldn't that be a negative velocity in your coordinates? Besides, the question asks for 'speed'.
     
    Last edited: Apr 11, 2007
  7. Apr 11, 2007 #6

    ssb

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    :!!) :rofl: :cry:

    Thanks!!!
     
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