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Homework Help: Newton laws and variable mass

  1. May 5, 2012 #1
    1. The problem statement, all variables and given/known data
    Water with mass density ρ are being shoot from a jet with cross sectional area A and velocity v0 at a man with mass m0.
    1. Find the force acting on a man at rest (assume all the water are absorbed in the clothes)
    2. Find the force acting on a man escaping at velocity v<v0.

    2. Relevant equations

    3. The attempt at a solution
    I calculated the momentum of a small water element dm = dmv0.
    this is the same as the momentum of the water and the man after the water hits him(no external forces):

    The momentum of the man only after the water hit him is: dp=m0dmv0/(m0+dm).

    But i couldnt get an expression of the force without dm or dt.
    Thanks for the helpers!
  2. jcsd
  3. May 5, 2012 #2


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    welcome to pf!

    hi itay2424! welcome to pf! :smile:

    (try using the X2 button just above the Reply box :wink:)
    why are you using dm ? :confused:

    the rate of momentum hitting the man can be calulated from ρ A and vo
  4. May 5, 2012 #3
    ok, so I replace dm with ρAv0dt. I still have the same problem.
  5. May 5, 2012 #4


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    force = rate of change of momentum …

    what's the problem? :confused:
  6. May 5, 2012 #5
    The problem is that I know the rate of change of mass but not the rate of change of velocity.
    Edit: nevermind I think I got it.
    Last edited: May 5, 2012
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