0
$\begingroup$

please I need clarification about the first principle of thermodynamics, it's general statement is:

$$\Delta U + \Delta \text{KE} + \Delta \text{PE}= W + Q .$$

Supposing that: $ΔU = 0$ and $Q = 0$, then: $\Delta \text{KE} + \Delta \text{PE}= W$ (of total forces).

But, We know from classical mechanics that :$\Delta \text{KE} + \Delta \text{PE}= W$(of non conservative forces)

We get : $W$(of non conservative forces)$= W$(of total forces)

WHICH IS ABSURD!

$\endgroup$
1
  • $\begingroup$ I've removed a number of comments that were attempting to answer the question and/or responses to them. Please keep in mind that comments should be used for suggesting improvements and requesting clarification on the question, not for answering. $\endgroup$ – David Z Jun 1 '20 at 19:26
1
$\begingroup$

Nothing is absurd here.

When writing $$\Delta U+\Delta \text{KE} +\Delta \text{PE}=W+Q$$ the work term $W$ is work done by non-conservative forces as well as work done by any external conservative forces you haven't included in $\Delta \text{PE}$. This is also true in your "classical mechanics" expression $$\Delta \text{KE}+\Delta \text{PE}=W$$

The $W$ here is not "net work done by all forces". This is only the case in your "classical mechanics" sense using the work energy theorem of $W_\text{net}=\Delta \text{KE}$. Where now you are talking about the work done by all forces.

So, really you have "W(of non conservative forces)= W(of non conservative forces)" which, of course, is fine.

$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.