Chronology | Current Month | Current Thread | Current Date |
[Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
From: phys-l-bounces@carnot.physics.buffalo.edu[mailto:phys-l->bounces@carnot.physics.buffalo.edu] On Behalf Of Leigh
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
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l