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Homework Help: Simple Dynamics question (just help to check the answer)

  1. Aug 19, 2012 #1
    Okay I have done it, but have a feeling that there's something wrong. I don't have the solution/answer so it will be great if someone can help me check. more of part 2 (which leads to part 3)

    1. The problem statement, all variables and given/known data
    a body of mass 6.0kg and density 450kg/m^3 is dropped from rest at a height of 7.5m into a lake. Calculate
    1. the speed of the body just before entering the lake
    2. the acceleration of body while its in the lake
    3. the maximum depth to which the body sinks before returning to surface.

    Neglect any surface tension or air resistance.

    3. The attempt at a solution

    sub s=7.5m, g=9.81, t=1.2365s
    then v=gt, v=12.131m/s

    2. Ffloat= ρgV, net force= Ffloat-mg
    V=6.0/450=1/75, net force=-(1000 x 9.81 x 1/75 -6.0 x 9.81)=-71.94
    then F=ma, a=-71.94/9.81=-7.3333m/s2

    for me, down is positive, up is negative

    3. using v2=v02+2as, v=0 when the body stops sinking.
    s=10.034m, which is the distance sank.

    Gahh I really think this question is rather easy, have not done dynamics for sometime, have given everything back to the teacher, sadly:(
  2. jcsd
  3. Aug 19, 2012 #2
    Why is this?
  4. Aug 19, 2012 #3
    ahh, it's you again:)

    Oh shoot wrong wrong it's a=F/m=-71.94/6.0=-11.99, so everything else follows is wrong. Thanks!
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