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Homework Help: Displacement from angle and initial velocity

  1. Oct 22, 2008 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown horizontally from the top of a building at 45.6 m/s. We want to know how long it takes for the ball to drop below an angle of 13 degrees. Ignore air resistance.

    2. Relevant equations

    I'm assuming the projectile equation is necessary (delta)y = Vyit + .5(a)t2 other than that I'm lost

    3. The attempt at a solution

    I've tried it a few different ways. First thought was to use the information given to write two equations. One being the flight path of the ball and the other of a line at 13 degrees. This gave me an incredibly small amount of time and was incorrect.
    The two lines would be -4.9t2 and -tan(13)x They both should intersect twice. Once being at zero the other the point we are interested in. This didn't work out.

    I tried rewriting equations to setup a different system of equations but ultimately couldn't get anything out of it.

    Somehow I got .55 and thought that was right but according to the teacher it is not. So what do I need to do to solve the problem???
  2. jcsd
  3. Oct 22, 2008 #2
    Any time an angle is mentioned, look for a right-angled triangle and components.

    Try drawing a sketch of what the projectile will look like at the exact moment in time you're after, spot a triangle, and label it with horizontal and vertical displacement components, plus any other information you know.

    It doesn't matter if you don't know values for the displacements yet - just use letters to represent the triangle's sides.

    Once you've done that, try to write down some equations for the horizontal and vertical displacements involved and see how that works out.
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