Re: Forces in rods, ropes, etc.

[LUDWIK KOWALSKI <kowalskil@alpha.montclair.edu>]

On Thu, 04 Sep 1997 Brian Whatcott was comparing a rod through which a 
constant force is applied to a brick with a coaxial cable through which 
the voltage from a battery is applied to a resistor. Brian wrote:

> It is a very easily arranged demonstration to suddenly apply a voltage
> from a battery to a coax transmission line. If there is a series resistor
> about equal to the characteristic impedence of the coax, typically 50 or 
> 75 ohms, in series with the battery, an oscilloscope will first show a 
> step from zero volts to HALF the battery voltage. Then when the steep 
> wave front has progressed to the (open ) far end of the coax, it travels 
> back at the level of the full battery voltage, finally reaching the 
> battery end. Its steady state is as an open circuit.

Why open circuit? A battery at one end, then the cable, and a resistor.
The stady state current is nearly V/R, unless the cable is very long. 
The matching of the R to the line's impedance is no longer significant 
in a steady state. 

Most people would accept the idea that "energy is flowing" along the
wire in a steady state. (Chemical energy is decreasing in the source 
while the internal energy is increasing in the receptor.) But they would 
probably reject a suggestion that "a force is flowing". 

According to the last message from Leigh, it is OK to say "a force is 
transmitted". Unless we agree that, by definition, only fluids (material 
substances which are not solids) can flow, I see no difference between 
"force transmitted" and "force flowing". Or, going away from physics, 
between "ideas transmitted" and "ideas flowing" (e-mail or otherwise). 

Leigh asked about the direction in which the force is transmitted along
a rod. I can ask the same for the electric line. In which direction are 
electric charges FLOWING? There are two current directions but only one
direction for the transfer of energy. 
                                                     Ludwik Kowalski

Yes, "thinking's fun", as you wrote, Leigh, and as confirmed by Margaret
on the other side of the globe. I wish my students would agree ...