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

[Jack Uretsky <jlu@hep.anl.gov>]

Hi Roger,
	Analogies, IMO, are for people who can't understand the real 
message.  I sometimes teach applications of Newton's laws, so I don't need 
any analogies.
	Please, what masses are you considering, ahd what forces?  Pls 
make yourself a free-body diagram, showing the masses involve and the 
gravitational force vectors acting, and then tell me how to reproduce your 
diagram.  This is essentially what would be required of your students in 
an AP exam.
				Regards,
					Jack

  On Tue, 25 Jul 2006, Roger Haar wrote:

> Hi,
>
> 	I have two very different suggestions for understanding this class of
> problems.
>
> 1.  Use the basic ideas of general relativity in which the "natural",
> force-free path in the presence of gravity is falling.  Then, if the
> rocket is hovering, it is actually accelerating upward.  ( This is a
> very simplified explanation. )
>
> 2.  Consider two aircarts connected by a spring.  The track runs from
> left to right and the spring is initially stretched so the carts would
> pull toward each other.  If one holds the left cart so that it is
> stationary in the lab frame, the right cart accelerates toward it.  Now
> replace the person holding the left cart with a rocket that keeps the
> left cart stationary in the lab frame. (This would require a controller
> on the rocket the adjusts the thrust constantly.) The right cart
> accelerates to the left and thus gains momentum, balancing the
> conservation of momentum equation.
>
> 	One catch with this type of problem is correctly apply Newton's Third
> law to gravity between two masses, but replace gravity with a spring and
> the action and reaction become at least somewhat clearer.
>
> Thanks
>  Roger Haar
>    U of AZ Physics
>
> Jack Uretsky wrote:
> > The change in momentum of the gas after it leaves the rocket cannot, at
> > leasst in first order, have any effect on the rocket.  I don't understand
> > your remark after the "but".
> >  			Regards,
> >  				Jack
> >
> >
> >
> > On Mon, 24 Jul 2006, Marc "Zeke" Kossover wrote:
> >
> > > --- Jack Uretsky <jlu@hep.anl.gov> wrote:
> > >
> > > > The answer to
> > > > what is getting
> > > > upward momentum is, simply, the earth.  The added
> > > > downward momentum from
> > > > the gas is from the gravitational attraction of the
> > > > each.
> > > This is what I originally thought (and the student, too)
> > > but it leaves out the fact that the rocket does get some
> > > positive momentum as well. The rocket end up upwards from
> > > it's original position, even though it remains the same
> > > distance from the planet.
> > >
> > > >  	I advocate keeping it simple and accompanying the
> > > > answer with a
> > > > diagram showing the gas, the earth, the mutual forces
> > > > acting on each, and
> > > > nothing else.
> > > Sure, but he was wondering about momentum conservation. And
> > > I think that he was leading me towards asking the more
> > > vexing question about momentum conservation in a plane with
> > > level flight.
> > >
> > > Marc "Zeke" Kossover
> > >
> > >
> > >
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