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Homework Help: Projectile thrown at a barrier

  1. Dec 14, 2009 #1
    1. The problem statement, all variables and given/known data
    A projectile is thrown from the horizontal ground with a speed of 40 m/sec at an angle with the horizontal of 30 degrees. At what height will the projectile strike a high barrier located at a distance of 85m from the firing point?

    2. Relevant equations
    ok so: point A is located at the point in which the projectile is launched
    point B is located at the point at which the projectile strikes the barrier
    point is located at the bottom of the barrier, on the ground directly below the striking point
    theta = 30 degrees

    change in time from A to B= [VB - VA] / acceleration

    dAB = distance from A to C * cos(theta)

    dBCx = VB (change in time from B to C) + 0.5a*(change in time B to C)2

    3. The attempt at a solution

    change in time A to B = [0 - 40] / 9.8 = 4.08 sec

    dAB = 85m * cos(30) = 22m

    dBCx = 0 (4.08 sec) + 0.5 (9.8) (4.08)2 = 245 m
    ^my teacher said that this wasn't right. i think i went wrong on the acceleration-i don't think i am calculating it right or using the right value.
  2. jcsd
  3. Dec 14, 2009 #2
    You're value for acceleration is fine in the y direction.

    How did you derive your equation for time?
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