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Homework Help: Finding velocity function from a model of force

  1. Feb 9, 2012 #1
    1. The problem statement, all variables and given/known data

    Hi, I've got an assignment which involves finding the function of velocity against time, given the information that the particle is moving with intial velocity of v0 and that it experience a resistive force of -βv2

    2. Relevant equations

    From the problem I've derived model of: ma = -βv2

    3. The attempt at a solution

    re-arranging the model, I have equation of:

    dv/dt = -β/m * v2

    then, using techniques from seperable equation,

    dv/v2 = -β/m * dt

    -1/v = -β/m * t + C1

    pluging in v(0) = v0

    -1/v0 = -β/m * 0 + C1
    C1 = -1/v0

    Now, plugging in C and rearraging the equation, I obtained

    -1/v = -β/m * t - 1/v0

    v = m/(β * t) + v0

    So as you can see, I'm able to derived the velocity function, but the problem lies to the fact that if I try to plug in t=0 for the velocity, then the function will not work as division by zero is not allowed

    Therefore, I'm thinking that something is wrong with my velocity function but up till now I can't still figure it out

    Thank you in advance!
  2. jcsd
  3. Feb 9, 2012 #2


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    Gold Member

    Welcome to PF!

    Your calculus is fine, you just forgot how to do division:[tex]\frac{1}{v(t)} = \frac{\beta}{m} t + \frac{1}{v_0}[/tex][tex]\Rightarrow v(t) = \left[ \frac{\beta}{m} t +\frac{1}{v_0} \right]^{-1} \neq \frac{m}{\beta t} + v_0 [/tex]
  4. Feb 10, 2012 #3
    Ah yes thank you very much at pointing out my mistake, guess I spend too much time thinking how to actually solve the differential equation that I forgot about that. In any case, thank you for helping me again :)
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