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Homework Help: At which times will the ball be at a height of 15 meters?

  1. Nov 29, 2006 #1
    1. The problem statement, all variables and given/known data

    I am in highschool physics. This is my problem that I've been trying to work on: A ball is thrown straight upward with a velocity of +20m/sec on a planet where the acceleration due to gravity is -10m/sec². Find the times at which the ball will be at a height of 15 meters.

    2. Relevant equations
    Ok, so I started with this equation: D=ViT+½at². I'm not sure if it's the correct one to use, but I'm pretty sure it is...I think I will need to use the quadratic forumla: x= -b+-b²-4ac (square root)/2a. Sorry, I don't know how to make a square root symbol or any of the other signs?!!?

    3. The attempt at a solution

    Now, the problem calls for time, so I must need to solve for it. Next, I got this as the possible equation to utilize when it's solved for time: t-v+- v²+f(1/2a)d (square root) / 2(1/2a)
    So far, does everything seem to be right, now all I need to do is "plug and chug?" :) Thanks
    -Greg
     
  2. jcsd
  3. Nov 29, 2006 #2

    OlderDan

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    Just take you equation for D and use the quadratic formula (or some other method) to solve for t when D = 15m. (Note that at t = 0, D = 0)
     
  4. Nov 29, 2006 #3
    S=ut + 1/2 at^2
    it should be used in a straight line motion and uniform acceleration.
     
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