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Projectile question

  1. Mar 30, 2016 #1
    1. The problem statement, all variables and given/known data
    Jim stands with a ball at the rear of a diving board, 10 m above the
    surface of a swimming pool. His friend Bob is swimming below, at a
    horizontal distance of 6 m from the front of the board. Jim thinks it
    would be funny to roll his ball along the diving board so that it drops on
    Bob’s head. Assuming that the effects of friction and air resistance are
    negligible, calculate the speed (in m s-1) at which Jim must roll the
    ball.

    2. Relevant equations
    s=u+1/2at^2
    speed=distance/time

    3. The attempt at a solution
    rearranged s=u+1/2at^2 to find t
    t=1.1s
    speed 6/1.1=5.45m/s
    However the answer is 4.20m/s
    where did I go wrong?
     
  2. jcsd
  3. Mar 30, 2016 #2

    SteamKing

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    Why don't you show us your calculations of how you obtained t = 1.1 s for the time it takes the ball to fall from the board and hit the water.
     
  4. Mar 30, 2016 #3
    I did square root of 2x6/9.81
     
  5. Mar 30, 2016 #4

    SteamKing

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    So the ball is falling horizontally? I thought the board was 10 m above the surface of the water.
     
  6. Jul 7, 2016 #5
    The equation is wrong

    The equation is s=ut+1/2at2

    The time taken for it to hit the pool when dropped is the same as when rolled since the horizontal and vertical components of the velocity do not affect each other.
     
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