Re: [Phys-l] Force on an accelerating piston

[John Denker <jsd@av8n.com>]

On 12/31/2006 10:40 AM, Dan Crowe wrote:

> If a rigid piston has mass, is the force that the gas exerts on the
> piston while the piston is accelerating away from the gas less than the
> force that the gas would exert on the piston if the piston was at rest?

1) I don't see any reason why the instantaneous acceleration of the
 piston should change the force impressed by the gas.  However, ....

2) To answer a slightly different question, the /velocity/ of the piston
 will change all sorts of things.
  -- This is especially obviously significant if the speed of the piston
   is not small compared to the speed of sound, but is still significant
   at smaller speeds.
  -- This is particularly easy to visualize if the piston is moving away
   from the gas: it will "outrun" some of the gas molecules.


There connects to some very practical real-world physics, including guns
and engines (steam engines as well as internal combustion engines, including
turbines).  The usual lame textbook discussion considers only the "ultra-low
RPM" limit of the operation of the engine;  however, to get any reasonable
power-to-weight performance from the engine, you need to operate it at a
high RPM ... the higher the better, other things being equal, but of course
other things are not equal, and finding out what factors limit the high-RPM
performance of the engine is an interesting and tricky business.


The case of a piston moving at supersonic velocity into a sample of gas is
the canonical way to introduce people to the idea of a /shock/.