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Finding Horizontal Distance in Projectile Motion

  1. Oct 18, 2009 #1
    1. The problem statement, all variables and given/known data
    A 0.5kg rock is projected from the edge of the top of a building with an initial velocity of 13.7m/s at an angle 60o above the horizontal. The building is 6.89m in height . At what horizontal distance, x, from the base of the building will the rick strike the ground? Assume the ground is level and that the side of the building is vertical. The acceleration of gravity is 9.8m/s2.

    2. Relevant equations
    dy = Voyt + 1/2at^2
    Vx = dx/t

    3. The attempt at a solution
    I have tried to find the vertical and horizontal component of the rock at its highest point. Using the vertical component, I found a time, which would be how long it took for the rock to get to its highest point from its original position. Then, I added the vertical component and the height of the building, and used that as the dy to find the time it takes for the rock to go from its highest point to the ground. I added the two times, and found a dx.

    My assignment is a quest on the UTexas site, so it would tell me when my answer is wrong.

    And here's a diagram:

    Thank you for your help!
  2. jcsd
  3. Oct 18, 2009 #2
    You're making this WAY more complicated than you need to. Did you learn that the x-component of velocity is Vcos [tex]\theta[/tex] and the y component of velocity is Vsin [tex]\theta[/tex], where theta is the angle made with the x axis?

    You're given everything you could possibly want in this problem, just plug and chug.

    Edit: To clarify, just write down what you do know. You know y-initial, y-final, x-initial, Vx initial, Vx final, Vy initial, acceleration in the x direction, acceleration in the y direction. The only things you don't know are time, x-final, and Vy-final. You have 3 unknowns and 4 equations (2 for x, 2 for y). Plug and chug.

    The way I do every one of these problems is to write down a list of everything you could possibly know about the system. That's 11 items. Initial and final position, initial and final velocity, acceleration, and time. Do that for the x and the y directions. Time is the same, so you only put that down once. After you list those 11 items, fill in what you know, and use that to solve for what you don't.

    If you do that, solving for it's highest point, or doing anything fancy like that doesn't even come into play.
    Last edited: Oct 19, 2009
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