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Homework Help: Projectile Motion

  1. Oct 6, 2011 #1
    1. The problem statement, all variables and given/known data
    A ball is projected vertically upwards from the top of a building that is 25 m high. After a time of 3 s it passes the edge of the building on its way down to the ground. In this question, g=10ms-2.

    a) What is it's initial velocity?

    2. Relevant equations

    s=so + vot + 1/2gt2

    v= vo + at

    3. The attempt at a solution

    Basically, all I want to know is this: Does the question want me to assume that it reaches the top of the projectile 'arc' where velocity in the vertical direction is zero, when the time is 1.5 seconds (half of 3 seconds). Or is there a mathematical way of showing it?

    I'd be fine if I knew one more variable, but I know very few variables...

    Thanks in advance.
  2. jcsd
  3. Oct 6, 2011 #2


    User Avatar

    Staff: Mentor

    It's not necessary to assume anything abut the time that it reaches the top of its arc (although you are correct that it will take half the time it takes to go from launch height to again be at launch height at the edge of the building).

    The first kinematic equation that you wrote is sufficient for your purpose. What's so? What's s when the ball passes the edge?
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