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# Physics questions - gravitational fields - hmmm watch

1. Mmm I am just looking through my revision guide for edexcel a2 physics (unit 5)

and on the bit about gravitational fields it says:

1. "Orbiting satellites have only the force of gravity acting on them."

I'm not being stupid, am I? Gravity isn't a force, right? It's the satellite's weight (F = mg) that's the force...

2. "An orbiting satellite..follows a line along a spherical equipotential surface"

Does this mean they can be elliptical, or not? (Im not too hot on geometry, but I don't think it does??) - If it means they can't be, is this also wrong?

My notes say that most natural satellites have elliptical orbits (whereas most artificial oens have circular orbits)- and I am sure this is true - e.g i think halley's comet has a very long thin elliptical orbit. I don't think all the planets have perfectly circular orbits either..
2. Yeah, the Weight is the force, not gravity - gravitational acceleration is an attraction that causes a force, but is also dependant on the mass of the satellite I think.
3. (Original post by mik1a)
Yeah, the Weight is the force, not gravity - gravitational acceleration is an attraction that causes a force, but is also dependant on the mass of the satellite I think.
just to clarify, you didn't mean that the gravitational acceleration was dependent on the mass of the satellite, right? You meant that the force/weight was?

rosie
4. Yeah I meant the force, that's really badly put now that I look at it.
5. to your first question. I'm pretty sure you can call gravity a force [a force field to be precise] (we refer to it as one of the 4 fundamental forces). Weight pretty much equivilent to "force of gravity".

In second question, equipotential is always in a spherical shape. However when we say equipotential, we mean "the same amount of gravitational potential energy". Comets and other stuff do not have a constant amount of gravitational PE, they slow down as they get further away, and speed up when they get closer. This is the same for all elliptical orbits i believe.
6. The weight is the force due to gravity between the satellite and the Earth. It is the force of gravity which causes the 'weight' (f=mg=GMm/r^2)
7. (Original post by Willa)
to your first question. I'm pretty sure you can call gravity a force [a force field to be precise] (we refer to it as one of the 4 fundamental forces). Weight pretty much equivilent to "force of gravity".

In second question, equipotential is always in a spherical shape. However when we say equipotential, we mean "the same amount of gravitational potential energy". Comets and other stuff do not have a constant amount of gravitational PE, they slow down as they get further away, and speed up when they get closer. This is the same for all elliptical orbits i believe.
Yeah, that makes sense - so all satellites do NOT have spherical orbits or constant potential energy, right?!

arrg - its so hard to know what you are meant to know - some things they just simplify and you have to say the simplified version to get the marks - others you are meant to know the "real" version?!

hmm it's just from years of teachers going "the force acting on it is NOT gravity, it's its WEIGHT"..
8. (Original post by rts)
The weight is the force due to gravity between the satellite and the Earth. It is the force of gravity which causes the 'weight' (f=mg=GMm/r^2)
Ok. I think that makes sense to me. It seems confusing that the force of gravity can both cause the weight and be the same size as the weight.. if that makes sense...but since the force of gravity depends on the mass too..

arrg what a tangle for my poor mangled brain

rosie
9. (Original post by crana)
Ok. I think that makes sense to me. It seems confusing that the force of gravity can both cause the weight and be the same size as the weight.. if that makes sense...but since the force of gravity depends on the mass too..

arrg what a tangle for my poor mangled brain

rosie
the weight is the force due to gravity
10. (Original post by rts)
the weight is the force due to gravity
you said "It is the force of gravity which causes the 'weight'" - so from what you said above "the force of gravity causes the force of gravity", or "the weight causes the weight" - I don't exactly see how something can cause itself, if you see what I mean? But you did have "weight" in quotes. I guess i know what you mean. I am just easily confused. Rosie.
11. 'Weight' is just the term for the force of gravity between an object and a planet.

F=mg is a simplification of the law of Universal gravitation (F = GMm/r^2), where g = GM/r^2. g is constant on the surface of the earth because G (a constant), M (mass of the earth) and r (distance from Earth's centre to object) are all constant at the surface of the earth.
12. (Original post by rts)
'Weight' is just the term for the force of gravity between an object and a planet.

F=mg is a simplification of the law of Universal gravitation (F = GMm/r^2), where g = GM/r^2. g is constant on the surface of the earth because G (a constant), M (mass of the earth) and r (distance from Earth's centre to object) are all constant at the surface of the earth.
okey dokey. thanks for your patience.

now......anyone else have any comments on orbits, elliptical or otherwise?

rosie
13. If a satellite does not have a spherical orbit, then it does not have a constant gravitiational PE
14. By spherical do you just mean the orbit has a constant radius.. cos spheres are 3d...
15. (Original post by crana)
Mmm I am just looking through my revision guide for edexcel a2 physics (unit 5)

and on the bit about gravitational fields it says:

1. "Orbiting satellites have only the force of gravity acting on them."

I'm not being stupid, am I? Gravity isn't a force, right? It's the satellite's weight (F = mg) that's the force...
It's a gravitation force that is dependent upon the satellite's mass. In A2 you get a whole topic on gravitational fields.

(Original post by crana)
2. "An orbiting satellite..follows a line along a spherical equipotential surface"

Does this mean they can be elliptical, or not? (Im not too hot on geometry, but I don't think it does??) - If it means they can't be, is this also wrong?

My notes say that most natural satellites have elliptical orbits (whereas most artificial oens have circular orbits)- and I am sure this is true - e.g i think halley's comet has a very long thin elliptical orbit. I don't think all the planets have perfectly circular orbits either..
Equipotential surfaces can be very irregular as they join the gravitational field lines that radiate out at right angles to the planet's surface. For modelling's sakes we assume it's spherical. In fact I lose exam marks if I don't draw my lines with a compass.
16. (Original post by Mysticmin)
Equipotential surfaces can be very irregular as they join the gravitational field lines that radiate out at right angles to the planet's surface. For modelling's sakes we assume it's spherical. In fact I lose exam marks if I don't draw my lines with a compass.
Isn't a planet regarded as a point mass (using it's center of mass) and so all equipotentials along it's gravitational field are spherical?
17. (Original post by mik1a)
By spherical do you just mean the orbit has a constant radius.. cos spheres are 3d...
yeah I meant like a ring, like as if it is going round the outside of a sphere. Sorry. I was quoting the revision guide when i said that - when i said it myself i said circular
18. (Original post by Mysticmin)
In A2 you get a whole topic on gravitational fields..
my goodness... this has come as a real shock. considering I'm revising that topic for a2 at the moment

(now you've made me paranoid that my complete ignorance makes me seem like a pre gcse student)

(Original post by Mysticmin)
Equipotential surfaces can be very irregular as they join the gravitational field lines that radiate out at right angles to the planet's surface. For modelling's sakes we assume it's spherical. In fact I lose exam marks if I don't draw my lines with a compass.
Fine. I'm not disputing that in any way!? All I was asking about was whether all satellites follow these (ROUGHLY circular, then!) orbits, as my revision guide says, or if some can be most definitely elliptical, as my notes say

rosie
19. (Original post by crana)
my goodness... this has come as a real shock. considering I'm revising that topic for a2 at the moment

(now you've made me paranoid that my complete ignorance makes me seem like a pre gcse student)

Fine. I'm not disputing that in any way!? All I was asking about was whether all satellites follow these (ROUGHLY circular, then!) orbits, as my revision guide says, or if some can be most definitely elliptical, as my notes say

rosie
Eeek, you're not ignorant, now you've got me panicking. I drew an asteroid as all bumpy for my synoptic paper, drew in the gravitational field in perpendicular and got big red crosses from my teacher.

In real life natural satellites do not follow roughly circular orbirts, however man made ones might have to...

Edit: for the sake of mr. edexcel I think we just call the satellite's weight force gravitational force, as otherwise you can't equate the whole centripetal force/gravitational force malarky.
20. (Original post by Mysticmin)
Eeek, you're not ignorant, now you've got me panicking. I drew an asteroid as all bumpy for my synoptic paper, drew in the gravitational field in perpendicular and got big red crosses from my teacher.

In real life natural satellites do not follow roughly circular orbirts, however man made ones might have to...

Edit: for the sake of mr. edexcel I think we just call the satellite's weight force gravitational force, as otherwise you can't equate the whole centripetal force/gravitational force malarky.
Yeah, my notes-from-my -teacher say that artificial satellites usually follow (roughly) circular orbits. It's just that my revision guide says all satellites do, but I'm sure not all natural ones do.

rosie..

arg for physics

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