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Power Law from ODE

  1. Aug 7, 2012 #1
    I'm trying to find a power law relationship between mass and metabolic rate, given that each of these quantities is defined by a differential equation.

    Assuming dM/dt=a*M(t) and dR/dt=b*R(t), where M(t) is mass and R(t) is metabolic rate, I know that I can solve each of these equations to get:
    M(t)=c*exp[at] and R(t)=k*exp[bt].

    Here is where the first part of my question comes in: Let's say I try to solve both of these for t, and then set them equal to each other. Then I end up with a*ln(M/C)=b*ln(R/K) where C and K are different constants than before, but that's not really important. Is solving for t and setting equal allowed? I'm not sure if I'm looking at a specific time where the two equations are equal in this case, but I can't think of any other way to get rid of the variable.

    Otherwise, I would think of dividing the two equations and getting M(t)/R(t)=h*exp[(a-b)t] where h is, again, a new constant that is unimportant. In this case, M=h*R*exp[(a-b)t]... which is different than when I eliminate t.

    In either case, I don't see a power law relationship! These are exponential, and not power law, equations unless I'm very mistaken. How can I get to the final product to see the power law in play?
  2. jcsd
  3. Aug 7, 2012 #2


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    Looks to me that c and k are the same as before, but a and b are inverted.
    Seems good to me. Why do you think it's invalid? You want to know the relationship between M and R at each given value of t.
    You don't?
    ln((M/C)a)=ln((R/K) b)
    (M/C)a=(R/K) b
  4. Aug 7, 2012 #3
    Thank you so much! Silly me- I was exponentiating both sides without bringing the constants into the log.

    I was unsure about solving for t because it seemed like I'm setting the two sides equal to each other for all t while they might not always be equal.
  5. Aug 10, 2012 #4
    Just divide one differential equation by the other to eliminate the dt.
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