Re: where does newtons laws fail

Gerry Quinn wrote:
> In article <1125735134.153451.18680@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> imsphy9@xxxxxxxxx says...
> > iam really searching for the answer where does newtons laws fail.i came
> > to know that in some cases without the application of force there may
> > be change in the momentum of the particle.how can this happen .can any
> > one quote the example.
> > what is a virtual work and how could it be possible.
> > thaks for ur valuable time
>
> Probably you are being confused by some poor quality explanation of
> some part of physics (could be one of several fields as far as I can
> see) that is using terms without explaining them properly, or how they
> relate to other theories.
>
> I *think* the discussion is probably about forces being explained by
> the exchange of virtual bosons. This doesn't actually mean Newton's
> law - or definition - f=ma fails at some point. It means it's being
> explained by a different model of how forces work. [I will also point
> out that a relativistic version of f=ma is still built into the new
> theory, even if strange things are done with it.]
>
> Let me give a simple example: I say "water doesn't evaporate, the
> faster atoms escape from the liquid". Am I saying that the ordinary
> theory of evaporation "added heat converts water into vapour with a
> certain latent heat of evaporation" is wrong or fails at some point?
> No - I'm explaining the same phenomenon in a different way.
>
> The term 'virtual work' actually indicates a bad jumble of unrelated
> concepts - we don't need the term work unless we are operating on a
> scale above that of virtual particle interactions. We can talk about
> momentum of virtual particles, but the only need for the term work is
> in the context of energy and entropy in thermodynamics, which is
> precisely to do with the scale in which the interference effects of
> virtual particles have been cancelled out, and real particles have been
> measured. And any real particle you measure will honour f=ma very
> well, allowing for a little residual quantum uncertainty that
> definitely can't be harnessed to do actual work.
>
> - Gerry Quinn

Is the question about the D'Alembert's Principle, isn't it?
Alain J. Brizard wrote a nice Introduction to Lagrangian and
Hamiltonian
mechanics:
http://academics.smcvt.edu/abrizard/Classical_Mechanics/CM.htm

.