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Homework Help: How to compute the magnitude of a initial velocity u

  1. Feb 17, 2017 #1
    1. The problem statement, all variables and given/known data
    A rock is thrown horizontally from a tower point A, and hits the ground 3.5 s later at point B. A line from A to B makes an angle of 50 degrees with the horizontal.
    Compute the magnitude of a initial velocity u of the rock.

    3. The attempt at a solution
    I started looking at the motion in x direction, which is
    u the unknown.
    Then i looked at the motion in y direction:
    And here we have the gravity.
    so i used the formula:
    s=1/2 * g * t^2

    1/2*9.81*(3.5)^2 =60 m/s^2

    from trigonometry

    solving x i get 50.34 meters

    But I'm not sure if I have done it corretly.

    Thank you for your help.
  2. jcsd
  3. Feb 17, 2017 #2


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    The units should be meters. Other than that, so far you are on the right track, but you have not answered the question yet. You found where the rock lands horizontally, but you are asked to find its initial speed.
  4. Feb 17, 2017 #3


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    That's right. But you need to find the x component of the velocity.
  5. Feb 17, 2017 #4
    aha, so I have to times the denominator with the time (3.5):

    Then I will get
    14.38 m/s

    It that true?
  6. Feb 17, 2017 #5


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    That is the correct answer. However it is more conventional to divide (untimes) the distance by the time to get the speed. :smile:
  7. Feb 17, 2017 #6
    haha, i will remember that for next time :) Thank you for your time. :)
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