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Confused whether question is right

  1. Sep 16, 2014 #1
    1. The problem statement, all variables and given/known data
    q)a rain drop of mass m is falling vertically through air with steady speed v the rain drop experiences retarding force kv,,k is constant,,acceleration of free fall is g,,k.e of drop is
    a)mg/k
    b)m.g^2/2k^2
    c)m^3.g^2/k^2
    d)m^3.g^2/2k^2
    e)m^2.g/k


    2. Relevant equations

    f=kv....k.e=0.5mv^2

    3. The attempt at a solution
    guys i ve solved by imagining that drop is in dynamic equilibrium
    f=mg
    kv=mg
    v=mg/k
    so opt d,,,,but since in question it is not written whether drop is in dynaimic equilibrium or not so i am confused
     
  2. jcsd
  3. Sep 16, 2014 #2

    nrqed

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    The question is stating that it is falling with a steady speed. This means that the speed is constant so yes, the net force is zero and your approach is correct.
     
  4. Sep 16, 2014 #3
    but after that it is written it is acted upon by force kv,,so it means that v would have changed ,,and it is not in eq for time being
     
  5. Sep 16, 2014 #4

    AlephZero

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    No, it means the retarding force = kv for any value of v. In the question there is just one constant value of v, which you found correctly as v = mg/k.
     
  6. Sep 21, 2014 #5

    rude man

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    It's a steady-state problem. When the drop begins to fall, v builds up as mg - kv = ma, a = acceleration, but then levels out where there is no further acceleration so mg - kv = 0.
     
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