Re: To hover, a reaction-motor pushes on the earth?

[Stefan Jeglinski <jeglin@4PI.COM>]

> If I eject one molecule downwards at high speed, I can expect it to
> share with two molecules at lower speed, and so on in increasing
> quantities until at appreciable distances beneath, it will be
> instrumentally difficult to detect the small pressure increment over
> the large area in question.


I find this thought experiment also helpful: consider yourself on the
standard frictionless rolling trolley or whatever, and propel
yourself by throwing balls off the back of it. If you consider
yourself and the earth below you as the complete system, momentum
conservation and the difference in mass ratio is all you need to
explain your brief acceleration and your resulting measurable speed.
When a ball leaves the hand, momentum is 'gained' due to the ejection
of mass. If in addition there is a rigid earth-connected wall to
throw against, when the ball hits that wall, it imparts yet another
(small) impulse to the earth which kicks it out a just little more
from under your frictionless trolley, which you actually interpret as
another increase in your speed.

The difference is slight but is a good way to see that there -is- a
difference between pointing the rocket exhaust at an object or into
say, deep space. This is a good pedagogical example, because once you
have convinced someone that the rocket is not moving by pushing
against the air, they often think that -nothing- behind the rocket
makes any difference ever, and once again you have to peel back
another layer.


Stefan Jeglinski