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Mechanics Question for a diver jumping off a Board

  1. May 28, 2012 #1
    1. The problem statement, all variables and given/known data
    A diver jumps off a 5m high diving board. The diver enters the water 1.5seconds later. Taking g=10m/s^2 determine
    (a) the speed he generates in his jump
    (b)his maximum height measured from the top of the diving board
    (c) the speed with which he enters the water


    2. Relevant equations

    v = u + at
    s = u + 1/2 a t^2
    v^2 = u^2 + 2as


    3. The attempt at a solution

    stuck on part (a) don't know if the force of gravity will be positive or negative since he is jumping.

    s = ut + 1/2 at^2
    s = 1.5(u) +1/2(-10) (1.5^2)
    5 = 1.5u - 5(2.25)
    5 = 1.5u - 11.25
    16.25 = 1.5u
    u = 10.83 m /s
     
  2. jcsd
  3. May 28, 2012 #2
    Your displacement equation is missing a crucial component: initial velocity. The "u" in your equation would represent distance, not velocity.

    The equation you need to use for the diver's position is:
    s=at2/2 + vit + si
    Where vi is initial velocity, and si is initial position. For this problem, you're given the initial position, so you just need to solve the equation for initial velocity, and plug in your known values.
     
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