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Homework: vertical motion in gravity

  1. Oct 2, 2006 #1
    i have this assignment. a ball is throun in to the air withe the speed equal to 49m/s and i need to find out after how many seconds it will reach 102,9 m. i used the formula we were given in class height=initial speed*time-[(Gravitational acceleration* time^2)/2]

    P.S. could you use these terms cuz i learn physics in romanian not in english
  2. jcsd
  3. Oct 2, 2006 #2
    i believe the equation is +at^2 and not -at^2....? alrite bascially u already have the ans so what seems to be the problem?the gravitational acceleration os 9.81....u just hav to sub in everything given to get the ans???
  4. Oct 2, 2006 #3


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    The part of the equation involving acceleration is 1/2*a*t^2 in general, but since a = -g, it is -1/2*g*t^2, so the equation is correct.
  5. Oct 2, 2006 #4
    The formula is
    x_f - x_i = v_0 t + \frac{at^2}{2}
    It comes from integrating acceleration as a function of time twice. If you know integration, then you don't need to memorize this formula, because it is so easily derived.

    You have all of the information; you have the formula as well. You are correct.
    102.9 - 0 = 49t - \frac{1}{2}(9.8)t^2

    Just remember that you need to use the quadratic formula to get the solutions from the quadratic equation that results from substituting in the numbers.
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