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Homework Help: Calculate Acceleration Given V0 and X

  1. Jan 23, 2012 #1
    1. The problem statement, all variables and given/known data
    An astronaut kicks a football on a level area of planet Zog. He kicks the ball repeatedly at a wide range of angles from nearly zero to almost 90 deg above the horizontal, but always as hard as he can which gives an initial velocity of 11.7 m/s. The astronaut notices that the maximum distance the ball lands away from him is 90 m. Armed with this information and because he mastered Physics 2048, he returns to his landing craft and quickly calculates the acceleration of gravity g on planet Zog. What is that value?

    V0 = 11.7 m/s
    X = 90 m
    X0 = 0 m
    Y0 = 0 m

    All of the relevant equations I think I can use involve time, which is not given. So, I do not know where to begin. If I can figure out where to start, I think I can figure it out pretty easily.
  2. jcsd
  3. Jan 23, 2012 #2
    At what angle will the ball travel the maximum distance?

    Think about that. The rest of the problem should follow naturally.
  4. Jan 23, 2012 #3
    You have to go a little bit outside of plugging values into the kinematic equations.

    Hint: The maximum distance the ball goes is 90m.

    EDIT: Sid beat me to it.
  5. Jan 23, 2012 #4
    Okay, that did get the ball rolling. I figured it was 45.0 degrees, so I calculated Vx and Vy.

    Vx = 7.78 m/s
    Vy = 7.78 m/s

    Then I plugged in these values to this equation to find t:

    X(t) = X0 + Vx0t
    90 = 0 + 7.78t
    11.6 s = t

    So then I plug t = 11.6 s into this equation:

    Y(t) = Y0 + Vy0t - 1/2(g)(t^2)
    0 = 0 + 7.78(11.6) - 1/2(11.6^2)(g)
    0 = 90.25 - 67.28g
    67.28g = 90.25
    g = 1.34 m/s^2

    But, this is incorrect. :[
  6. Jan 23, 2012 #5
    Are you sure you calculated the initial velocities correctly?
  7. Jan 23, 2012 #6
    No I just realized that. For some reason I was putting 11 instead of 11.7 - silly me.

    With new numbers, I got it right. Thanks guys!
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