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Is this test question wrong?

  1. Mar 11, 2012 #1
    Hi, I think the sample physics problem given by the ADA on their website is wrong. Can anyone confirm? It says: A ball is thrown horizontally at 20 m/s from the top of a building and strikes level ground 50 m from the building. How high is the building?

    When I solve this, I get 62m, but the answer is 31m. I assume the discrepancy is the horizontal velocity. I'm assuming it is constant while maybe it should be an average? Can anyone help me confirm this? I thought I had a strong hold on kinematics and I'm worried I'm getting stuck on this problem.

    Thank you, anyone!
  2. jcsd
  3. Mar 11, 2012 #2
    The answer on the website is correct, and the horizontal velocity is meant to be constant. It seems you do not use the correct equation.
  4. Mar 11, 2012 #3
    The time of flight, assuming contant horizontal velocity, is:

    t = 50m/(20 m/s) = 2.5s.

    Using s = u*t + 0.5 * a * t^2 you get 31 m.

    Getting 62m looks like you forgot the 0.5.
  5. Mar 11, 2012 #4


    User Avatar

    Staff: Mentor

    My guess is you used the vertical velocity at impact instead of the average vertical velocity...
  6. Mar 11, 2012 #5
    Yes, thank you for pointing out I used the wrong equation. I did indeed leave out the 1/2. Ironically I am relieved and can move on!

    You guys are so great :)
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