Re: [Phys-l] Rocket Hovering and Conservation of Momentum

[John Denker <jsd@av8n.com>]

Marc "Zeke" Kossover wrote:

> The gas gets negative momentum and the rocket
> gets positive momentum due to conservation of momentum, but
> the rocket is pulling on the planet 
...
> What say you?

I don't have much to add, except to strongly recommend this method
of analysis.

In a simple case like this, there are lots of equally-workable
methods of analysis ... but in more complicated situations,
this flow-of-momentum approach is the winner, hands down.

Momentum is conserved, so you know before you begin that in
any steady situation, momentum is flowing around and around in
a closed circuit.  The task, then, is just to find all the
elements of the circuit.

Similar situation (airplane rather than rocket):
  http://www.av8n.com/how/htm/airfoils.html#sec-air-momentum

Additional applications of the momentum-flow idea:
  http://www.av8n.com/physics/euler-flow.htm
  http://www.av8n.com/physics/pressure-everywhere.htm

There is also the "Newton's Cradle" demo where momentum is lost by
ball #1,   flows _through_ balls #2, #3, and #4, leaving them unchanged,
and is finally imparted to ball #5.