Re: Conservation of angular momentum

On May 30, 9:13 am, "Androcles" <Engin...@xxxxxxxxxxxxxxxx> wrote:
"Peter" <Poakfi...@xxxxxxx> wrote in message

news:1180529487.991933.155140@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
: On May 29, 5:53 pm, "Androcles" <Engin...@xxxxxxxxxxxxxxxx> wrote:
: > "Peter" <Poakfi...@xxxxxxx> wrote in message
: >
: >news:1180466839.198558.180620@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
: > : On May 29, 2:50 pm, "Greg Neill" <gneill...@xxxxxxxxxxxxxxx> wrote:
: > : > "Peter" <Poakfi...@xxxxxxx> wrote in message
: > : >
: > : >news:1180464002.678171.224180@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
: > : >
: > : >
: > : >
: > : >
: > : >
: > : > > On May 29, 1:49 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:
: > : > > > On May 29, 10:07 am, Peter <Poakfi...@xxxxxxx> wrote:
: > : >
: > : > > > > On May 29, 10:44 am, "Greg Neill" <gneill...@xxxxxxxxxxxxxxx>: > wrote:

: > : >
: > : > > > > > "Peter" <Poakfi...@xxxxxxx> wrote in message
: > : >
: > : > > > > >news:1180448943.790902.171370@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
: > : >
: > : > > > > > > As I understand it, Lagrange and Hamilton did not invent
new
: > : > physics;
: > : > > > > > > everything they said can be derived from Newton's laws of
: > motion.
: > : > But
: > : > > > > > > I see an incompatibility between torque r x F and Newton's
: > second
: > : > law
: > : > > > > > > F = dp.dt. In Newton's equation, there is no room for an
r.
: > : >
: > : > > > > > F = dp/dt p is linearmomentum
: > : >
: > : > > > > > T = dL/dt L isangularmomentum
: > : >
: > : > > > > I know that. The problem I have is that
separateconservationlaws
: > for
: > : > > > > linear andangularmomentumconflict with reality. I saw a device
: > : > > > > (which I understand will soon be on the market), consisting of
a
: > : > > > > closed air track, with roughly the shape of a horseracing
track,
: > : > > > > where it is clearly seen linear andangularmomentumreadily
convert
: > : > > > > into each other. This conflicts with two
separateconservationlaws.
: > : >
: > : > > > Peter, we've talked about this before, but you seem to have
: > forgotten.
: > : > > > In your analysis of this oval track, you *switch* the origin by
: > which
: > : > > > theangularmomentumis defined. This isn't legal, nor does it make
: > : > > > physical sense. Please refer to the earlier discussion to locate
: > your
: > : > > > error. I'm not particularly interested in rehashing endlessly an
: > error
: > : > > > you refuse to correct.
: > : >
: > : > > > PD- Hide quoted text -
: > : >
: > : > > > - Show quoted text -
: > : >
: > : > > I have not forgotten anything, and I do not switch the point of
: > : > > reference, which is at the center of the track. The track is twice
as
: > : > > long as it is wide, theangularmomentumof the puck when it starts
at
: > : > > the center of the track is r x p, and when it is at one end of the
: > : > > track is 2r x p. The linear (tangential) speed of the puck is the
same
: > : > > at all times. No torque acts on the puck at any time.
: > : >
: > : > You've ignore the fact that the track is not the only
: > : > part of the system. It's fixed to the Earth, which
: > : > must participate in the reactions. Your chosen center
: > : > is not inertial.
: > : >
: > : > Try the example again with the track sitting on a
: > : > frictionless surface (like an air table). As the puck
: > : > rounds the ends, the track will translate and rotate.- Hide quoted
: > text -
: > : >
: > : > - Show quoted text -
: > :
: > : Of course that Newton's third law holds at all times. That is why the
: > : Earth oscillates: the Moon revolves around it. Nevertheless, it is
: > : considered that theangularmomentumof the Moon is conserved.
: >
: > That's where you screw up, Pokefun.
: > The axis spinangularmomentumof the Moon alone is (almost) conserved.
: > Theangularmomentumof the Earth-Moon system is conserved.
: >
: > This happens,
: > http://antwrp.gsfc.nasa.gov/apod/ap051113.html
: > proving (I said "proving") the Moon is in elliptical orbit and the
: > spinangularmomentumof ONE body is NOT conserved.
: >
: > Much as the Moon keeps one face toward the Earth, as the Earth slows
: > to eventually keep one face toward the Moon (long after we are all
: > departed this mortal coil) the Earth will lose it's own
spin(angularmomentum)
: > and "days" ( time spent facing the sun) will be longer than they are
now.
: > But theangularmomentumof the SYSTEM will be conserved.
: > This means that the Moon will gain some of the Earth's spin and move
: > further away into a more circular and less elliptical orbit, as most of
the
: > planets have already done. The comets are in highly elliptical orbits
: > but are relatively short lived. Halley's comet was not at all
spectacular
: > the last time around.
: > It also means that the entire solar system will attempt to spread
theangular
: > momenta evenly throughout, and why Mercury, closest to the Sun,
: > has the greatest eccentricity of all the planets.
: >
: > That is the genius of Newton. He saw.
: > He stood on the shoulders of giants and saw further than any
: > man before him. He is STILL the giant.
: >
: > You discuss linearmomentum. Ok, but remember that velocity
: > has a sign, mass does not. Mv + mV is conserved.
: > When one ball of mass m moves with velocity V to strike
: > another ball of mass M, velocity zero, the effect is identical
: > to mass M moving with velocity -v and striking mass m which
: > has velocity V.
: > Learn the PoR.
: > http://www.androcles01.pwp.blueyonder.co.uk/PoR/PoR.htm
: >
: > : After
: > : all, it has been going around for a long time.
: >
: > Not an argument. The Moon didn't always keep one face toward the Earth.
: > It lost it's own spin until it reach equilibrium. The Earth-Moon system
: > is still not yet in equilibrium. Earth is gradually slowing down.
:
: I suppose what you say is true. It seems you specialize in astronomy.
:
: Peter

The sky is the biggest laboratory you'll ever find, Peter, and astronomy
the oldest science. You are quite correct thatmomentumis conserved,
what you gain on the swings you lose on the roundabouts. The sum
total of all momenta in the Universe is zero, as it is in any closed system.
Two bodies, initially at rest with respect to each other as in
this double ended cannon, have equal and opposite momenta for a
total of zero.
http://www.androcles01.pwp.blueyonder.co.uk/MC2.htm

I agree completely.

Peter


.

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