 |
|
|
|
|
|
| |
| |
|
|
|
|
| |
| |
|
|
>> Because you need angular *acceleration* to "rob the planet of momentum",
>> not
>> angular velocity.
>
>> Simply spinning at a constant 5000rpm is not going to change the planet's
>> momentum, it will stay constant no matter how many times you turn or for
>> how
>> long you stay turning.
>
> Take into account air friction and friction from the ground. In order to
> maintain the constant turning speed you have to actually accelerate
> constantly due to these frictions.
>
> Basically you can't have anything connected to the plane rotating at
> a constant velocity without applying a force to it (because of friction).
> And an applied force means acceleration.
Friction against what? Newton's 3rd law...
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
scott <sco### [at] laptop com> wrote:
> Friction against what? Newton's 3rd law...
Against the ground and the air.
Friction causes heat, and the energy for that heat must come from somewhere.
I cannot be produced from nothing.
The answer is that the energy which produces that heat is coming from
the angular momentum, which is thus reduced by a proportional amount.
--
- Warp
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Darren New wrote:
> Orchid XP v7 wrote:
>> There's 300 enemy troops with SMGs, grenades and rocket launchers, not
>> to mention 3 helicopters, and yet 1 lone commando manages to shoot
>> them all down without dying?
>
> That's what always un-immerses me in FPS games. In Quake II (IIRC) you
> wind up basically killing the entire enemy army yourself without ever
> seeing another living teammate.
An entire army is sent to the alien homeworld. The aliens wipe out all
but 1 soldier, who is left armed with only a tiny pistol. Yet he alone
is able to destroy an entire string of tactically significant targets,
completely unaided.
Hmm. :-)
I guess the idea is to massage your ego by letting you be the
super-soldier that nobody can kill.
Neat intro tho...
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Warp wrote:
> I cannot be produced from nothing.
That's deep...
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
> Warp wrote:
>
>> I cannot be produced from nothing.
>
> That's deep...
:D
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Warp wrote:
> scott <sco### [at] laptopcom> wrote:
>> "Each turn robs the planet of angular momentum"
>
>> Unfortunately not...
>
> I don't see, technically speaking, why not.
>
> Let's assume we have a big object in a weightless space in vacuum
> rotating. Can this rotation be stopped by the object itself without
> applying external forces?
>
> The answer is yes. The most obvious way to do this is to fire up some
> rockets in specific directions. This causes the rotation to slow down,
> basically by expelling material at high speed from the object.
>
> A slightly less obvious way is to rotate some significant part of the
> object at a different speed. Why does this slow down the overall rotation?
> It's because part of the angular momentum is converted into heat due to
> friction, and this heat dissipation is taken away from this angular momentum.
>
Nope.
As the part is spun up, the larger part is spun in the opposite
direction to a degree determined by their relative masses. The net
angular momentum of the system stays constant assuming that this is a
closed system.
When the spin is removed as you suggest by friction, slowing the
'rotating part' applies an equal but opposite force to the main body.
This results in the assembly as a whole returning to the original rate
and direction of spin (or rest). It can however be left pointing in a
different direction.
Angular momentum is conserved.
The heat dissipated is simply the energy used to spin up the system
being returned.
What is useful is that the system - say a satellite - can be pointed to
and kept at different directions and that unwanted rotation can be
countered by precisely controlling a number of gyroscopes.
Real satellites are subject to external forces like drag, outgassing and
uneven light pressure that can give them unwanted spin. Gyroscopes can
be used to soak this up. Eventually if the external forces keep
building up in the same direction the gyroscopes can't be spun up safely
any more to counter the rotation. The satellite operator uses thrusters
to really counter the rotation with off-centre thrust. At the same time
the gyroscopes are spun down and returned to a rate of rotation that is
within a comfort zone and correct to match any residual spin after the
thruster stops.
When the gyroscopes seize or when the thruster propellant is exhausted
the satellite could no longer be controlled. Usually some time before
this the satellite is retired to a safer parking orbit or in some cases
is even de-orbited. The idea is to do this while the operator can
control the orientation.
Hubble is a good example. It has 6 large gyroscopes. Originally 3 were
used to be able to point and hold very steady in any given orientation.
The others were provided as spares. Quite a few gyroscopes have
failed and had to be replaced during servicing missions. IIRC it was
out of action for a while when 4 gyros failed but they developed a way
to operate with only 2 but with reduced capability. At the moment it is
operating with close to no redundancy and NASA has concluded that a
shuttle won't be used to perform another service mission. The proposal
was to de-orbit it while they have control. This caused furore and NASA
was reconsidering. Last I heard a 5th servicing mission was back on for
next year. Pessimistically I'd guess that no mission will happen and
that good old Hubble is either going to be brought down or otherwise
become unusable when the next gyro fails.
Another aside - There is some research going on to develop
micro-thrusters that could potentially do away with gyroscopes in future
satellites. Each one provides a once-off and very small amount of
thrust. A very large number are built into a grid. Imagine something
like a chip built using semiconductor fabrication holding 1000x1000
little electronically fired cells. Each contains a microgram of solid
propellant. Place lots of these chips at strategic points on the
outside of the satellite.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Paul Fuller <pgf### [at] optusnetcomau> wrote:
> As the part is spun up, the larger part is spun in the opposite
> direction to a degree determined by their relative masses. The net
> angular momentum of the system stays constant assuming that this is a
> closed system.
You assume that the rotation can be done without any friction. This is,
in fact, impossible in practice.
Friction produces heat. Heat is energy. This energy must come from
somewhere.
Even if the Earth-Moon system was a completely isolated closed system
in space, Earth's rotation would still slow down. Why? (Granted, the
situation is not identical, but the basic cause for the slowdown is.)
--
- Warp
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Warp wrote:
> Paul Fuller <pgf### [at] optusnetcomau> wrote:
>> As the part is spun up, the larger part is spun in the opposite
>> direction to a degree determined by their relative masses. The net
>> angular momentum of the system stays constant assuming that this is a
>> closed system.
>
> You assume that the rotation can be done without any friction. This is,
> in fact, impossible in practice.
No I don't. Friction is not the issue. Wherever the energy comes from
- say a battery or solar panel - is irrelevant. Where it goes to - heat
via friction or some of it converted back into electricity - doesn't matter.
>
> Friction produces heat. Heat is energy. This energy must come from
> somewhere.
Energy was stored in the rotating masses. You stop them rotating then
you get the energy back in one form or another. But you can't just stop
one of the masses rotating. There is an opposite effect on the other mass.
>
> Even if the Earth-Moon system was a completely isolated closed system
> in space, Earth's rotation would still slow down. Why? (Granted, the
> situation is not identical, but the basic cause for the slowdown is.)
>
If you are referring to the cartoon then the Earth would be affected
because a small part (the girl) starts rotating by pushing against it.
The Earth's rotation is altered ever so slightly in the opposite
direction. While she spins at a constant rate the effect on the Earth
stays the same. Since she will experience friction she has to add
energy to keep spinning at the same rate. However even if she
experienced lower friction or none at all it does not matter to the
rotation so long as she adds energy to stay rotating at the same rate.
When she stops spinning either by allowing friction to do its job or by
actually exerting a force herself then either way a force acts
ultimately on the Earth in the opposite direction.
There is a simple statement that you either agree with or not: "Angular
momentum in a closed system is conserved". Yes or No ?
If 'No' then please provide an explanation or link explaining how any
form of energy can be turned into angular momentum in a closed system.
A Nobel prize in physics and immense wealth awaits.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Paul Fuller <pgf### [at] optusnetcomau> wrote:
> > You assume that the rotation can be done without any friction. This is,
> > in fact, impossible in practice.
> No I don't. Friction is not the issue. Wherever the energy comes from
> - say a battery or solar panel - is irrelevant. Where it goes to - heat
> via friction or some of it converted back into electricity - doesn't matter.
In order for the rotating secondary object to affect the primary object's
rotation, it has to be connected to the primary object somehow. This
connection causes friction.
> > Friction produces heat. Heat is energy. This energy must come from
> > somewhere.
> Energy was stored in the rotating masses. You stop them rotating then
> you get the energy back in one form or another. But you can't just stop
> one of the masses rotating. There is an opposite effect on the other mass.
This would be true in a completely friction-free system. The thing is,
friction dissipates part of this energy.
> > Even if the Earth-Moon system was a completely isolated closed system
> > in space, Earth's rotation would still slow down. Why? (Granted, the
> > situation is not identical, but the basic cause for the slowdown is.)
> >
> If you are referring to the cartoon then the Earth would be affected
> because a small part (the girl) starts rotating by pushing against it.
> The Earth's rotation is altered ever so slightly in the opposite
> direction. While she spins at a constant rate the effect on the Earth
> stays the same. Since she will experience friction she has to add
> energy to keep spinning at the same rate. However even if she
> experienced lower friction or none at all it does not matter to the
> rotation so long as she adds energy to stay rotating at the same rate.
I have no idea what cartoon you are talking about.
Anyways, in the Earth-Moon system (and in fact, in any planet-moon
or star-planet, or basically any object-orbits-another-object system)
the slowdown of the rotation of the Earth is caused by tidal forces
caused by the Moon. In practice it means that the Moon deforms the
Earth as the Earth rotates, and this deforming produces heat (because
of friction) which is dissipated. This heat energy is "robbed" from
somewhere: The angular momentum of the Earth.
It's the reason why the Moon always shows us the same side. It has not
always been like that, but it has become like that because of tidal forces
slowing it down.
In the Pluto-Kharon system this is even more accentuated, as they both
orbit each other synchronously, each one showing the other always the
same side. It has not always been so.
So, you see, even in a closed system rotation can be stopped without
ejecting any material, just by converting angular momentum into heat.
> There is a simple statement that you either agree with or not: "Angular
> momentum in a closed system is conserved". Yes or No ?
Angular momentum can be lost by converting it to heat (or other forms
of energy for that matter), so the answer is no, unless you don't consider
it a "closed system" anymore if there's heat dissipation (OTOH, this heat
could theoretically be collected and stored, keeping the whole thing a
closed system).
> If 'No' then please provide an explanation or link explaining how any
> form of energy can be turned into angular momentum in a closed system.
How do you think things started rotating in the first place?
And what is your explanation of why planets and moons are slowing down?
--
- Warp
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Ok, here's an easy way of stopping the rotation of an object in a closed
system:
Construct a cannon on the surface of the object so that it will shoot
in the direction of the tangent, towards the rotation direction. Shoot
a heavy-enough object fast enough so that the recoil of this shot will
stop the rotation of the main object. (If you are worried about possible
effects of having just one such cannon on the surface of the object then
just put another identical cannon at the opposite side of the object,
aimed towards the opposite direction, ie. still at the direction of
rotation at that side, and fire both cannons simulatenously.)
The projectile is attached to a cable connected to the object at a
point where the pull caused by the object when it reaches the cable
length causes an even force on the center of mass of the object. (Such
a point must exist because there are two extremes: Attaching the cable
on the object at the point where the cannon is would effectively negate
the whole operation. However, attaching the cable on the opposite side
of the object would actually make it start rotating in the opposite
direction compared to the original one. A balance point must thus exist
somewhere in between.)
When the projectile has thus been stopped, just pull it back to the
surface of the object. The rotation of the whole system will have stopped
without permanently ejecting any material.
(Where did all the angular momentum go? Well, stopping the projectile(s)
will require energy. The majority of this energy will be dissipated as
heat from the cable.)
--
- Warp
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
