Re: [Phys-l] basic laws of motion +- vectors +- angular momentum

[John Denker <jsd@av8n.com>]

On 12/16/2008 10:32 AM, Bill Nettles wrote:
> Then there's the whole problem that if you take two moving point charges
> (q1 and q2) with non-parallel velocities (v1 and v2), Newton's 3rd Law
> doesn't work for the Lorentz forces they exert on each other if you
> calculate the B-field using the Biot-Savart Law. The force on q2 will
> have a component in the v1 direction and the force on q1 will have a
> component in the v2 direction....impossible to be opposite in the
> general case.
>  
> Is there a way to make N3L work in this classical situation?

That's certainly a "classic".  Feynman asks the very same
question in volume II, near the end of section 26-2.  Another
example of the same funny business, perhaps easier for students
to visualize, is embodied in the clever machine shown in
figure 17-5.

The answer is given in section 27-6.  The third law of motion
is tantamount to conservation of momentum ... nothing more,
nothing less.  And there is momentum in the fields.

You don't need to remember the section numbers I just cited.
It suffices to look up "Poynting" in the index.

Trying to formulate the third law overly narrowly -- e.g. in 
terms of forces on _particles_ -- is a losing proposition.