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Homework Help: Motion in 2D jumping

  1. Feb 4, 2008 #1
    1. The problem statement, all variables and given/known data
    A man jumps from the top of a building to the top of another building 3.7 meters away. After a running start he leaps at an angle of 14 degrees with respect to the flat roof while traveling at a speed of 5.1 m/s. The acceleration of gravity is 9.81 m/s/s. To determine if he will make it to the other roof, which is 1.1 meters shorter than the building from which he jumps, find his vertical displacement upon reaching the front edge of the lower building with respect to the taller building.

    2. Relevant equations
    y = v[tex]_{}oy[/tex]t - (1/2)gt[tex]^{}2[/tex]
    x = v[tex]_{}ox[/tex]t

    3. The attempt at a solution
    3.7 = 5.1cos14t
    t = 3.7/5.1cos14
    y = 5.1sin14(.7) - (1/2)(9.81)(.7[tex]^{}2[/tex])
    y = -1.54

    The answer sounds reasonable - what could possibly be wrong?
  2. jcsd
  3. Feb 4, 2008 #2


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    Homework Helper

    I don't see anything wrong with your approach. I notice that you do a lot of rounding of numbers in between calculations. This will affect your final answer slightly. Also, have you taken into account that the 2nd building is 1.1m shorter than the 1st?
  4. Feb 4, 2008 #3
    Just out of wonderment what answer did the book give you?

    Merely glancing over your work and not solving it myself. I think one problem is your 51.sin14.7 part. If I am reading this right, the other roof is shorter than the roof you are on, since your displacement is downwards which means your velocity is downwards, which means it should be negative.
  5. Feb 4, 2008 #4
    What should be negative? I gave the answer y = -1.54, and I actually have tried several other answers close to that. Also, I'm not using a text book; I'm entering answers into an online homework service, so I don't know what the right answer is until I figure it out myself.

    Any idea what specifically I should change?
  6. Feb 4, 2008 #5
    What exactly do I need to do with the 1.1 difference in heights? If anything, I thought that would be the displacement.
  7. Feb 4, 2008 #6
    Your Vo should be negative.
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