Angular Momentum - help please

[Zeffer]

I have just finished reading BANG - my wife's Christmas Pressy to me!

Great book but wondered if anyone could help me get my head round this. Does the energy from angular momentum stay the same and this has a bearing on the energy that is being put out by pulsars and a bearing on why the moon is moving 4cm away from the earth each year? Can someone explain this in really easy terms?

[Sam]

Zeffer,

I no astrophysicist but here's my laymans take on what's going on. Not sure about the pulsar bit but if you've got something whizzing around the earth (like the moon) then it's traveling at a certain speed for it's orbit but the same angular speed (ie degrees per hour) for any orbit it's in. If the moon slows down it has to move closer to the earth to maintain the same angular speed (angular momentum as well as it's mass is the same in both instances). But if the moon speeds up then it has to go to a higher orbit to keep the angular speed the same (of course the km/sec speed (tangential velocity?) is a heck of a lot faster). I saw a thing on Sky the other day saying the same thing you read about the moon moving away at approx 4cm a day, I can't remember why it was doing that.

hope that helps

cheers

Sam

[Astroman]

For the pulsar bit, no, angular momentum isn't related, at least directly, to the energy released. Pulsars are formed by massive stars that generally undergo a core collapse supernova. The original star can be many times the mass and size of our Sun, but rotate much faster. (Sol rotates once in 33 days at the equator and 26 days at the poles.) When the star collapses and blows off its outer layers, the angular momentum of rotation must be conserved. This makes the remaining material "spin up" to a very high speed, sometimes hundreds of revolutions per minute. Pulsars emit energy in a focused beam that radiates from fixed spots around the equator, much like a light house. We detect their energy as they "sweep" across us. The energy is not related to the conservation of angular momentum, but our detection of it is.

[Paxo]

I have just finished reading BANG - my wife's Christmas Pressy to me!

Great book but wondered if anyone could help me get my head round this. Does the energy from angular momentum stay the same and this has a bearing on the energy that is being put out by pulsars and a bearing on why the moon is moving 4cm away from the earth each year? Can someone explain this in really easy terms?

Zeffer,

I must agree with you, a great book.

With respect to the increase in distance between the Earth and the :moon:, by the action of tugging on the tides (frictional forces of the water on the Earths surface), the Moon is decreasing the angular velocity (rotational speed) of the Earth (This happens over a very long time scale hence the small change of distance , a few cm's per year). Conservation of angular momentum of the Earth-Moon system says that the Moon must gain angular momentum to keep the overall energy of the system the same i.e its orbit shifts outwards to compensate, for as already pointed out, higher orbits have a greater velocity.

To complicate things there are also changes in potential and kinetic energies in the system due to tidal bulges...., but you did say in easy terms....the earths rotation is slowing, so to keep the energy in the system the same (Newtons laws) the moon speeds up (in terms of orbit not rotation), as it sppeds up it moves further away from the Earth.

A bit tekkie, but I hope this helps.

Regards Steve.

[Zeffer]

Thanks for all the help - I have re-read the passages and it seems to be coming together. It was the fact that angular momentum has to be preserved that I didn't take in initially - it all makes sense now

I totally muddled the pulsar bit - it was all to do with their speed!

[themos]

This may be useful:

http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit4/tides.html