Solving Differential Equations: Understanding Variable Changes

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Discussion Overview

The discussion revolves around the differentiation of equations, specifically the application of variable changes in the context of differential equations. Participants explore the validity of differentials derived from equations involving multiple variables and the implications for integration and differentiation.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant questions whether both df = m da and df = a dm are valid computations when differentiating the equation f = ma, and whether this flexibility applies to equations with three or more variables.
  • Another participant confirms the validity of the "rocket equation" derivation, indicating that df = m da/dt + a dm/dt is a correct interpretation.
  • A different participant provides a generalization for differentiating a product of multiple variables, stating that df = ab dc + ac db + bc da, emphasizing the differentiation of each variable individually.
  • One participant seeks clarification on the flexibility of substituting variables for integration, asking if df = m da could be used to integrate with respect to a, and similarly for other variables, while recognizing the product rule in their reasoning.
  • Another participant notes that the derivative of a product of N variables results in N terms, each differentiated once, providing an example with r and theta.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding regarding the application of differentiation rules and the flexibility of variable substitution. There is no clear consensus on the extent to which these substitutions can be applied in different contexts.

Contextual Notes

Some participants mention the product rule and its implications, but there are unresolved questions about the assumptions underlying variable substitutions and the conditions under which they hold true.

bassplayer142
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A little thing that may be stupid but I am confused about it. Say we take any equation like f=ma
If we take the derivative of both sides then we could either have

df=m da
or
df =a dm

Are both of these valid computations. If I am looking to change the integrating variable can I use this any way I want? And would this work with any equation relating 3 or more variables?

thanks in advanced
 
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You are basically correct: the "rocket equation" comes from F = dp/dt = d(mv)dt = m dv/dt + v dm/dt.

For what you wrote, f = ma, df = m da/dt + a dm/dt.
 
If f=abc, df=ab dc+ ac db + bc da, and so on for an number of variables.
You just differentiate one at a time.
Of course if any factor is a constant, then its differential is zero.
 
Andy Resnick said:
You are basically correct: the "rocket equation" comes from F = dp/dt = d(mv)dt = m dv/dt + v dm/dt.
The "rocket equation" is a bit different, because the exit velocity of the gas enters instead of just v in the dm/dt term.
 
thanks this clears some up. But as I said before could I take df=mda to subsitute df with da to integrate with respect to a, and in the same problem could I take df=adm to integrate with respect to m.

so you could take s=rTheta and make it ds=rdtheta?

I'm just seeing how flexible I can be when substituting vaiables to integrate or differentiate with.

Edit, I just realized that what you did there was the product rule which makes sense. Is what I just said above wrong then?>
 
Last edited:
A derivative of all product of N variables will have N terms, each term being differentilated once. So d(r theta)=r dtheta+theta dr.
 

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