Thermal Energy Equation Term - Chain Rule

kevman90
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Homework Statement


I am going through a derivation of the thermal energy equation for a fluid and am stumped on one of the steps. Specifically, the text I am using converts the term:

P/ρ*(Dρ/Dt)

to:

ρ*D/Dt(P/ρ) - DP/Dt

where:
ρ = density
P = pressure
D/Dt = material derivative

The text says this is done using the chain rule of differentiation but I can't derive it myself. I'm far removed from calculus so maybe I'm missing something simple but any help would be appreciated.

Homework Equations

The Attempt at a Solution

 
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One of the two expressions you have needs an extra minus sign. Momentarily, I will show the calculus of the second expression with the chain rule... @kevman90 Do you know how to take the derivative of ## \frac{d(uv)}{dt}##? It is ## u (\frac{dv}{dt}) +v(\frac{du}{dt}) ##. In this case, ## u=P ## and ## v=1/\rho ##. With the chain rule, ## \frac{dv}{dt}=(\frac{dv}{d \rho}) (\frac{d \rho}{dt}) ##. Do you know how to compute ## \frac{d v}{d \rho} ## ? With that, you should be able to process the second expression that you have, but I think you will find that it equals the minus of your first expression.
 
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Charles Link said:
One of the two expressions you have needs an extra minus sign. Momentarily, I will show the calculus of the second expression with the chain rule...
Sorry @Charles Link -- I was in the process of deleting the OP and warning for not showing enough work. But if you want to give a couple hints, that's probably okay.
 
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This makes sense - didn't think about using the product rule. I will work through it later but I think I've got it. Also my mistake with the minus sign I forgot to include it out in front of the first term. Thanks!
 
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Prove $$\int\limits_0^{\sqrt2/4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx = \frac{\pi^2}{8}.$$ Let $$I = \int\limits_0^{\sqrt 2 / 4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx. \tag{1}$$ The representation integral of ##\arcsin## is $$\arcsin u = \int\limits_{0}^{1} \frac{\mathrm dt}{\sqrt{1-t^2}}, \qquad 0 \leqslant u \leqslant 1.$$ Plugging identity above into ##(1)## with ##u...

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