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Homework Help: Review Q #3

  1. Sep 21, 2010 #1
    Problem solved! Thanks again Thaakisfox!

    1. The problem statement, all variables and given/known data
    A ball thrown horizontally at 2.2 m/s from the roof of a building lands 36 m from the base of the building. Calculate the height of the building.
    Vox = 2.2 m/s
    Xo = 0m
    Vx = 0 m/s
    X = 36m
    2. Relevant equations
    y = Vosin ao - (0.5)(g)(t^2)
    3. The attempt at a solution
    I tried plugging it in but then I have a missing time...?
     
    Last edited: Sep 21, 2010
  2. jcsd
  3. Sep 21, 2010 #2
    This is a horizontal projection hence you dont need any sine etc. Just calculate the time of descent from the velocity and horizontal displacement by dividing them. And then the vertical component is simple freefall.
     
  4. Sep 21, 2010 #3
    I'm kinda confused.
    What formula would I use then?

    x=Vo *CosAo * t
     
  5. Sep 21, 2010 #4
    No. It is a horizontal projection. You dont need any of those sine or cosine function.

    You throw the ball with horizontal velocity Vo. Since it has no acceleration in the horizontal direction, the distance it travels is simply: x=Vo*t where x is given (the distance from the base of the building). from here you can get t. Now in the vertical plane it is just simple freefall, so use the formula which gives the distance when freefall takes place
     
  6. Sep 21, 2010 #5
    36 m = 2.2 m/s * t

    36/2.2 s = t
    16.36 s = t

    Formula for free fall: h(t) = Vo*t + 1/2 at^2
    h(16.36) = 0*t + (0.5)(-9.8m/s^2)(16.36^2)
    = -1311.48304 m ?

    I think something went wrong... :(
     
  7. Sep 21, 2010 #6
    The calculation is correct. The given data seem insensible.
     
  8. Sep 21, 2010 #7
    Insensible?
     
  9. Sep 21, 2010 #8
    That happens many times in textbooks, that the given data for a problem dont make sense. For example there probably arent many buildings taller than 1km, and especially someone throwing a ball off.
    But your calculation is correct. (That minus sign doesnt matter, it just means that you took the y axis to point upwards, and thats why the acceleration has a negative sign. But you are searching for the absolute value of the height anyway, so just take that minus sign away.)
     
  10. Sep 21, 2010 #9
    Oh! Good observation.

    Problem solved! Thanks again Thaakisfox!
     
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