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billybojoe08
2004-Apr-21, 12:11 AM
What are the three laws of planetary motion :roll: :(

Normandy6644
2004-Apr-21, 12:44 AM
Do you mean Kepler's laws?

bmillsap
2004-Apr-21, 12:46 AM
Google 'Kepler laws' and you'll find plenty of sources. :)

Lorcan Faol
2004-Apr-21, 12:46 AM
Kepler's Laws.

1. A planet's orbit is an ellipse, with the Sun at one focus.
2. An imaginary line from a planet to the Sun covers the same amount of area in space over the same amount of time, anywhere on the orbit. (I can't think of a better way to word this one, right now)
3. p^2 = a^3
p- planet's sidereal period in years
a- planet's semimajor axis, in AU

Is that what you are asking for?

daver
2004-Apr-21, 12:53 AM
Kepler's Laws.

1. A planet's orbit is an ellipse, with the Sun at one focus.
2. An imaginary line from a planet to the Sun covers the same amount of area in space over the same amount of time, anywhere on the orbit. (I can't think of a better way to word this one, right now)
3. p^2 = a^3
p- planet's sidereal period in years
a- planet's semimajor axis, in AU

Is that what you are asking for?

You could restate these as

1. Gravity obeys an inverse square law
2. Angular momentum is conserved

ToSeek
2004-Apr-21, 01:01 AM
I hope we're not doing someone's homework for them.

Lorcan Faol
2004-Apr-21, 01:07 AM
2. Angular momentum is conserved

That seems to be a little vague, though, when not explaining how/why/when it is conserved...

Ut
2004-Apr-21, 03:29 AM
2. Angular momentum is conserved

That seems to be a little vague, though, when not explaining how/why/when it is conserved...

Why: Because there are no external torques on the system.
When: Whenever there are no external torques on the system.
How: Strangely enough, I don't understand the question...

Lorcan Faol
2004-Apr-21, 05:36 AM
2. Angular momentum is conserved

That seems to be a little vague, though, when not explaining how/why/when it is conserved...

Why: Because there are no external torques on the system.
When: Whenever there are no external torques on the system.
How: Strangely enough, I don't understand the question...

Well yeah, I know why and when (forget the "How?", I was just throwing that in as an extra, but unneeded, interrogative for no real reason #-o ). But I just meant that stating Kepler's second law as simply "Angular momentum is conserved" would make less sense to someone not very familiar with these laws, or with physics in general. I think the way I tried to explain it, had it been a more clear, and better worded explanation, would have made more sense to someone who was new to these concepts.

StarStuff
2004-Apr-21, 05:47 AM
I hope we're not doing someone's homework for them.

My thoughts exactly.

TriangleMan
2004-Apr-21, 11:48 AM
I hope we're not doing someone's homework for them.
It would be a pretty straightforward thing to Google so it's best that he get the answers from knowledgable people - you never know what kind of weird site you might find on the net with incorrect info.

AstroCreep
2004-Apr-21, 12:17 PM
I hope we're not doing someone's homework for them.

I think you are...Same person asked homework related questions in this thread amazingly the questions were also about Kepler

And was told by the BA to research on his/her own.

iFire
2004-Apr-21, 12:41 PM
Those "Interview Questions" look more like questions on a worksheet or something... :-s.

StarStuff
2004-Apr-21, 01:43 PM
I hope we're not doing someone's homework for them.

I think you are...Same person asked homework related questions in this thread amazingly the questions were also about Kepler

And was told by the BA to research on his/her own.

And he has now asked yet another question in a thread entitled "help". What's more, despite getting thorough answers from some of the board members in this topic and the original Kepler one, he has not once bothered to come back onto those topics and discuss them further, or even thank anyone for the help. :-?

Lorcan Faol
2004-Apr-21, 03:38 PM
#-o #-o #-o :roll:
What a punk...

Glom
2004-Apr-21, 03:56 PM
Almost a welcoming place, isn't it? :roll:

mike alexander
2004-Apr-21, 04:25 PM
Perhaps a little slack should be cut for someone in 6th grade who wants to know how modern astronomical concepts evolved, eh? While I agree with the BA that we shouldn't do someone's homework for him (her?), we might also congradulate billyboetc for seeking out independent sources of information (he/she still has to put it all together).

A very good source for info on Kepler is Arthur Koestler's book The Sleepwalkers. It can be read by a 6th grader who is willing to work at it. Kepler comes off as a very sympathetic figure.

The first law (elliptical motion) was forced on Kepler by his analysis of the motions of the planet Mars, based on Tycho's (Tighe) observational data, although it took a tremendous shift in thinking to break away from circles, epicycles and deferents (note to billy: look these up, and find out what they mean).

The second law says that if you draw a line from a planet to the sun, the area that the line sweeps over will be equal for the same amount of sweep time (Note to billy#2: draw some ellipses using cardboard, two tacks, some string and a pencil. Look up how to do this. Change the spacing of the tacks to see how it affects the shape of the ellipse. An interesting thing to include in your report.)

The third law says that if you take the time required for a planet to go around the sun and square that number, it will be proportional (be careful about saying equal!) to the semimajor axis of its ellipse cubed (multiplied by itself three times). Look up what a semimajor axis is before you write anything. You can go to many astronomical reference books or web sites and get the values for the period of the planets and their distance from the sun. Make a table and actually do the calculations (very easy on a calculator) and see for yourself if the law really works.

Oh. Say thanks to all the folks here who are helping you.

Ut
2004-Apr-21, 04:56 PM
I was trying to remember where I read a little about JK's life, and I think it was in The Discoverers by Daniel Boorstin. I don't think it touched on the actual science, though. Any 6th grader should be able to read it, despite it being something like 700 pages. The bit about him should be near the back. Any public or high school library should have a copy. It's also going for \$1.25 used on Amazon.

And frankly, it doesn't sound so much like the kid's filling out a worksheet as it does s/he's trying to get some tidbits of information to throw down on a little wall display. End of the year "science" report project style.

daver
2004-Apr-21, 05:16 PM
But I just meant that stating Kepler's second law as simply "Angular momentum is conserved" would make less sense to someone not very familiar with these laws, or with physics in general. I think the way I tried to explain it, had it been a more clear, and better worded explanation, would have made more sense to someone who was new to these concepts.

I agree. In fact, I was trying to find a way to say that in the post you quoted, but couldn't figure out a good way to word it. Keppler's laws are quite useful; the fact that you can derive them from more fundamental relationships doesn't change that at all.