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Homework Help: Projectile motion equations

  1. Sep 12, 2016 #1
    1. The problem statement, all variables and given/known data
    A ball is tossed from an upper-story window of a building. The ball is given an initial velocity of 7.90m/s at an angle of 21.9deg below the horizontal. It strikes the ground 2.95s later.
    Calculate the height from which the ball was thrown.
    2. Relevant equations

    3. The attempt at a solution
    y=y0+vy0t- 1/2gt^2
    vy0=sin21.9 deg*7.9m/s=2.9466m/s
    could anybody tell me where have done wrong???
  2. jcsd
  3. Sep 12, 2016 #2


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    Check your signs.
  4. Sep 12, 2016 #3


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    You need to be consistent about how you assign the positive directions. If you want positive to be up, what is the sign on y0? If you want positive to be down, what is the sign on the acceleration?
  5. Sep 12, 2016 #4
    Thank you:)
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