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Re: [Phys-l] The "why" questions



On 11/30/2010 11:07 AM, Stefan Jeglinski wrote:
/snip/
2. Like some others, I am squarely in the camp of F(t) = ma(t), where
t is t. To me, any non-philosophical discussion of causality would
require consideration of integral formulations of F(t) ~ ma(t-t') or
a(t) ~ (1/m)F(t-t').
/snip/
we
are again led to a similar painful discussion! To wit, does voltage
"cause" current or is it the movement of charge that "causes" a
potential difference to be created? I =want= to identify this with
the F(t) = ma(t) discussion but I'm not so sure it's as clear.

Stefan Jeglinski

Allow me to offer concrete examples of two easily demonstrated
set ups.

If I suspend a lab weight on an elastic band, then place the upper end of the band in movement - perhaps with constant velocity upwards, or apply a constant force upwards with rather different effects, then I expect the suspended weight to trail the motion of the upper end and show some oscillation en route to steady motion. Here f(t) and m. a(t) seem not to describe a symmetrical function.

If I connect a battery to a lengthy coax - open circuit at the far end: the coax consumes energy as though it were a resistor of the coax characteristic impedance - 50 to 80 ohms as it might be.
After a time defined by twice the coax length and a large fraction of c, that energy flux stops.


These are not painful instances but they do have practical implications.
Are these instantiations of what you have in mind here?

Brian W