mistakey on the csu site? (1 Viewer)

[Rahul]

hmm....take a look here...

i think that they forgot to include the radius of earth into consideration.
if it says that it is orbitting at x km, do we assume that earth's radius is included.

i'm pretty sure that the radius of the earth has to be included in these calculations.

 

[Lazarus]

No; you don't need to include the radius of the earth. Gravity is caused by the mass as a whole, not simply the point at its centre.

Or, for an alternative explanation, you could exchange the two masses in the equation, and you certainly don't include the radius of the satellite!

[Edit: See later post for amendments.]

 

[Rahul]

ok, but when you look at the defintion of Newtons law of universal gravitation:

The two factors that determine the force of the attraction are:
- the mass of each of the two objects
- the disticances between their centres of mass.
~taken from the csu site

now when you have to calculate the gravitational force between earth and a satellite, wouldnt the radius of earth have to be included, as per the definition above?

 

[Lazarus]

Hmm. I've changed my mind (though not because of your points ).

There's obviously a force of attraction between a person standing on the earth and the earth itself (i.e. your weight). You should be able to calculate it. If you don't include the radius of the earth, and the person is on the earth's surface, r = 0, which obviously cannot work with the equation.

But, going back to your first example, by the same token, if you include the radius of the earth, you should be including the radius of the satellite (which you never do). Perhaps it's never done because the difference is negligible. Shrug.

I guess I stand corrected.

 

[Rahul]

yes, that is correct:
W=mg

g=Gm/r^2, where r is the radius of earth.

i doubt that a mistake has been made by csu.

Originally posted by Rahul
if it says that it is orbitting at x km, do we assume that earth's radius is included.

maybe i should look at the term orbit and see if it linked to the centre of mass of the system.

 

[Lazarus]

If you assume that a planet is a sphere with uniform density, its centre of mass will be at its centre.

 

[Rahul]

that can never be the case, in theory. there are oil and natural gas reservoirs that will lead to an inconsistent gravitational field (the last dot point i did )
but for the syllabus, we consider the planets to be perfect!

 

[ssssonicyouth]

what dot point would that be?

 

[Rahul]

its the 2nd dotpoint on the left hand column and also the 2nd on teh right in section 3 of space.

 

[BlackJack]

Hmmm... on the other hand if csu says the near-earth orbit satellites are <6400km then they did not include radius into the equation. And this question is wrong.

If it's <6400km then they did include it, because near earth orbits are on the magnitude of 100's km if I remember correctly

 

[Rahul]

yes, satellites can orbit earth at an altitude of ~300 to ~800 km. and earth's radius is 6400km. so its very likely that the question is 'correct' and the radius is included.

maybe the term orbit, takes into account the radius of earth.

 

[Antisocial]

Yes, I remember my teacher hammering this into our head during one of the earlier lessons:

- orbital radius = radius of earth + altitude.
- the radius of the orbit is sum the radius of Earth and its altitude.
- the altitude is the height of the orbit above ground.

So the term orbit already takes into account the radius of the Earth and altitude...

I think.

 

[Rahul]

ahh! bingo!

thought it would have been the case...

cheers~

 

[jims]

nah i still reckon their working is wrong and agree wif u that there should be an extra 6400km in their value for r.
their value of 35,800km is just the altitude because in the jackaranda textbook (pg 49), they work out r from kepler's 3rd law using the mass of the earth and the period as a side-real day. they get r as ~42,000km and then subtract the radius of the earth to get 35,800km.

 

[xiao1985]

nah i recon the question, if taken in a different way is not wrong (tho i admit the wordin is confusin):

"... a satellite, orbitin at the distance of 35800 km ... " it doesn't specify 35800 km from where?? however, since the two objects we are talkin about here are earth and satellite, then it is natural to assume that 35800 from earth to satellite. this effectively is the center of gravity of the earth to the center of gravity to the satellite...

altho officially, geosyncrhonise satellite has the altitude of ~36000km...... so yeah, the either the question / answer is wrong, or they are talkin abt a different earth...... =)

 

[stag_j]

the csu site has a lot of mistakes. mainly mathematical ones with the working. i emailed them and told them about the ones i'd picked up on and they have since been fixed. but because of this i'd suggest that if you see something on this site that doesnt quite look right, chances are its the site thats wrong, not you

 

[Wahlito]

Do people recommend using the notes on this site to study from?

Some of my friends say that its good because its direct answers to syllubus dot points, but others say the notes are incomplete and u need to know more.

 

[Antisocial]

IMHO, these extra notes would be better used as supplementary material (adding to your complete set of notes) rather than complementary material (heavily relying on them). You'll find you understand and remember concepts much better (I sound like a teacher).

 

[Rahul]

use those notes to either make your notes[which you should have already, unless you are like me] or to check that you have covered the right material for a certain dot-point. use it as a template to make sure that you are/were on the right track.