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



Note: In my last posting I wrote about the [exhaust gas plus rocket fuel] subsystem. I was thinking of solid rocket "fuel", of course. For those who care, modify this subsystem to include the oxidizer (or hypergolic propellants - whatever it takes) thus use: "the [exhaust gas plus rocket fuel plus oxidizer] subsystem".

My intercalary comments on this morning's Digest follow.

Leigh

On 28-Jul-06 Jeff Schnick wrote:

I don't see any problem with treating momentum as a transferable vector
physical quantity.

Neither you nor I will have a problem, Jeff. That is because we are grounded firmly in the fundamental principle of conservation of translational momentum. Our students, for the most part, are not so firmly grounded. Zeke's student is not yet ready for this limited extension of the principle. He indicates this by asking a perfectly reasonable question. I don't think I have seen a reasonable answer to that question in terms of momentum as a transferable vector quantity presented here, at least in terms the student in question would really understand.

I don't think that the fact that the magnitude and direction of the
momentum that one observes a particle to have depends on the inertial
reference frame from which one observes the particle makes momentum any
less real. After all, the magnitude of a static electric field at a
particular point in space depends on the inertial reference frame from
which one observes the electric field and that doesn't make the electric
field any less real.

We disagree on that point. I will present my argument on the reality of the electric field in another thread if you wish. As just a hint, let me point out that it can be transformed away at a point in space by a frame change!

Also, the fact that the earth's gravitational field is continually
transferring momentum to the exhaust gas of the rocket naturally
accounts for the fact that the earthward directed momentum of the
rocket's exhaust gas continually increases until the exhaust gas
collides with the earth. It doesn't indicate that momentum is not a
real entity.

Here I disagree again. I think that including "earth's gravitational field" as an element in your model system is incorrect. That, too, is in no respect a real entity. I note that there is a conspicuous inertial frame in which it does not exist, but I have other arguments against the reality of the gravitational field per se.

In physics we aim merely to describe natural phenomena as accurately as possible. In the pursuit of this goal our science has proved spectacularly successful. It seems that the frontiers of ignorance are being pushed back at a satisfyingly rapid pace

I will add that we physicists should not pretend that we are "explaining" anything. We may safely leave that to the religious, recognizing that their approach may be sterile, but it keeps them happy if we leave them alone.

Jeff Schnick

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l->bounces@carnot.physics.buffalo.edu] On Behalf Of Leigh
Palmer
Sent: Wednesday, July 26, 2006 8:24 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] Rocket Hovering and Conservation of Momentum

On 25-Jul-06, at 9:00 Zeke Kossover wrote:

Howdy-

A student asked me this question recently, and I am not
sure about my answer.

He asked, "Imagine a rocket hovering off the ground,
applying a thrust equal to the rocket's weight. Gas is
moving downwards getting downwards momentum. What is
getting upwards momentum?"

I haven't read any of the ensuing discussion, but I will give my
completely predictable response to this question. I will note in
passing that the hovering rocket is Jonathan Archibald Wheeler's
favorite example of the principle of equivalence, and mine as well.
Without looking out the window, an astronaut aboard the rocket cannot
tell the difference between his situation in Earth's gravitational
field and that of an astronaut in outer space in an accelerating
rocket producing the same thrust. The forces acting upon the
astronauts are the same in both cases, though some misguided folk
would characterize one of those forces as "fictitious".

Oh yes, my answer: have your student apply the principle of momentum
conservation in the context to which it is constrained. The momentum
of an isolated system is conserved. In this case the phrase "getting
downwards momentum" implies that momentum is a real entity, a common
cognitive error.

Momentum is not a real thing. It should not be looked upon as being
acquired in this case. As your student quite correctly observes and
illustrates, that idea is sterile. It leads to no physical
enlightenment; it is just confusing. We should listen more carefully
to our students' questions. In many cases they may have been lead to
the glib application of concepts with which they do not fully
understand and appreciate. Rather than constructing elaborate
justifications for misapplying such concepts we might better serve
them by returning to the basics.

Two things are evidently wrong with the idea that the momentum of the
exhaust gas is a real thing. In the first place, the momentum of this
particular gas must be defined relative to some particular frame of
reference. That is not the case, for example, with a real physical
entity like electric charge or mass. Also one must marvel as well at
the fact that the "downwards momentum" of the gas seems to increase
with time as the gas descends in Earth's gravitational field. Odd
behavior for a real entity.

One must go through the analysis systematically: define the system,
assure that it is isolated, and calculate the momentum of the system
before and after a given process. If you have chosen wisely you will
get the same answer for each calculation. Sounds dull, perhaps, but
it is a very powerful tool.

I will not extend this already prolix note farther because there is
no need to. Do the analysis!

Leigh