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Homework Help: Projectile motion 2-d kinematics

  1. Sep 24, 2011 #1
    This is an even problem from Cutnell & Johnson 8 edition. Can anyone verify if it is correct?

    In the annual battle of the dorms, students gather on the roofs of Jackson and Walton dorms to launch water balloons at each other with slingshots. The horizontal distance between the buildings is 35.0 m, and the heights of the Jackson and Walton buildings are, respectively, 15.0 m and 22.0 m. Ignore air resistance.

    (a) The first balloon launched by the Jackson team hits Walton dorm 2.0 s after launch, striking it halfway between the ground and the roof. Find the direction of the balloon's initial velocity. Give your answer as an angle measured above the horizontal.

    (b) A second balloon launched at the same angle hits the edge of Walton's roof. Find the initial speed of this second balloon.


    t = 2 seconds
    x = 35 meters
    y = -4 meters

    Vox = x/t = 35/2
    Vox = 17.5 m/s
    Vox = VocosƟ
    17.5 = VocosƟ
    Vo = 17.5/cosƟ

    Voy = Vo + at

    y = Voyt + 0.5at2 = -VosinƟ -4.9t2
    -4 = (17.5/cosƟ)sinƟt -4.9t2 <----plug in t = 2
    -4 = (17.5)(2)tanƟ - (4.9)(4)
    Ɵ = 24.0 degrees

    Vo = 17.5/cosƟ
    Vo = 17.5/cosƟ = 19.2 m/s

    ahh i forgot to do part B.....But the answer is 28.9 m/s for part B?
    Last edited: Sep 24, 2011
  2. jcsd
  3. Sep 24, 2011 #2


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    Nice method of calculation!!
  4. Sep 24, 2011 #3
    WOOHOO!! Thank you! I love this site....My teacher refuses to give us the answers for even problems, even right before the quiz! How are we supposed to know if we are doing it right :(
  5. Sep 24, 2011 #4


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    If you think you have the right answer, you are most probably right. If you are not sure whether you are right, you are quite possibly wrong. CONFIDENCE! - backed up by sound preparation.
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