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Homework Help: Gravitational Motion

  1. Sep 8, 2009 #1
    1. The problem statement, all variables and given/known data

    An object is thrown downward with an initial speed of 19 m/s from a height of 85 m above the ground. At the same instant, a second object is propelled vertically from ground level with a speed of 54 m/s .

    The acceleration of gravity is 9.8 m/s^2 .

    At what height above the ground will the two objects pass each other? Answer in units of m.

    2. Relevant equations

    Distance Fallen By Obj @ Free Fall= (gt^2)/2

    3. The attempt at a solution

    I honestly don't know how to start off besides setting two sides of an equation equal to each other and finding a common variable.
  2. jcsd
  3. Sep 8, 2009 #2


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    Science Advisor

    The equation you list is valid for an object with zero initial velocity, which neither of your objects have.

    Are you familiar with the basic kinematic equations for constant acceleration which relate acceleration, initial velocity, time, distance, and final velocity?
  4. Sep 8, 2009 #3
    I'm familiar with all these equations.

    The problem is actually very easy to visualize and understand, but the numerical work can't be done without the proper equations

    Vf=Vo + gt where Vf is final velocity and Vo is 0 (at the top of the projectile curve) although this also isnt very useful

    Vavg= (Vf-Vo)/2


    but i honestly dont know what to work with. thus, i'm asking for help

    please explain how to solve this question
  5. Sep 9, 2009 #4


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    Science Advisor

    The relevant equation is

    [tex]y=y_0 + v_{0y} t + \frac{1}{2}a_{y}t^2[/tex]

    For some initial height, vertical velocity, and vertical acceleration. In applying this equation, be sure to define a coordinate system and keep all your signs consistent!
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