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Homework Help: Tricky kinematic problem.

  1. Feb 10, 2005 #1
    Hi all, can some one please help me on this problem;

    The acceleration of a marble in a certain fluid is proportional to the speed of the marbles, and is given (in SI units) by a= -3.00V^2 for v>0. If it enters this fluid with a speed of 1.50 m/s, how long will it take for the marble to reach half its intial speed.


    I have released that

    Vf=Vi + a(t)

    .75=1.5+ a(t)
    -.15=a(t)


    and that A=(Vf-VI)/t

    I am stuck after this. Please help me.
     
  2. jcsd
  3. Feb 10, 2005 #2

    dextercioby

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    Science Advisor
    Homework Helper

    Do you calculus??This problem involves solving a first ODE through separation of variables...

    Daniel.
     
  4. Feb 10, 2005 #3
    I am still stock on this problem. I have tryed using seperation of variables but it just leads me in a circle.
     
  5. Feb 10, 2005 #4

    Doc Al

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    Staff: Mentor

    [tex]\frac{dv}{dt} = -3 v^2[/tex]
    [tex]-\frac{1}{3 v^2} dv = dt[/tex]
    Integrate!
     
  6. Feb 10, 2005 #5
    I can't beilve I did not see that :eek: LOL, its soo easy. Thank you for your help.
     
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