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Homework Help: Problem concerned to kinematics !

  1. Jun 24, 2012 #1
    1. The problem statement, all variables and given/known data

    A body standing on a bridge throws a stone vertically upwards with a velocity of 20 m/s. it strikes the water below after 3 seconds, the height of the bridge over the water level in the river is :
    a. 45 m
    b. 10 m
    c. 5 m
    d. 15 m
    2. Relevant equations

    vf^2= 2aS + vi^2

    3. The attempt at a solution
    its ans mentioned in key is 5m, but i dnt undrstand how !!
    The time taken to reach max height (h) is given by..
    v = u - gt .. where v = 0 m/s
    t = u/g .. .. 20m/s / 9.80m/s² = 2s ..

    So it takes 2x2s (4s) to return to the position it was thrown from (on the bridge) .. yet the question gives a time 3s to reach the water below the bridge.
  2. jcsd
  3. Jun 24, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You're right. It makes no sense. (Must be an error in the problem statement.)
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