Originally Posted by

**lonelyoutpost** Thanks for your help. I think maybe I shouldn't have used the word 'equation' in my original question. I realise that the maths works, that the two sides of the equation are equal and that a=b and therefore b=a.

What I should have said from the start is...

The actual equation I saw was:

dV/dt = a

and then the author said to multiply both sides by V giving:

d/dt (V^2)/2 = av

I was confused at that point because I couldn't see why the LHS wasn't written:

(dV/dt) . V

The logical jump to reverse the chain rule just hadn't occurred to me. And it still wouldn't (although I can see why it works). I think the problem for me is that I haven't yet grasped a deep, intuitive feel for what the chain rule actually means.

Anyway, thanks all for your help. I need to give it some thought. If someone can explain the answer to my question (which I find very difficult even to word right) I'd be interested to hear from you!