Okay here is something else I have pondered in my bored times. Maybe someone can shed some light on this one as well.

Everything in the universe is set on an orbital track. We spin and rotate around our sun, predictably in the same manner every year. However, gravity is effected by mass. If the mass of our planet changes, how does that effect our orbit? We've launched many things into space, thus [/b]very minutely[/b] changing the mass of our planet. However, over the speed and distance our planet travels year by year, a small value can snowball into a huge effect. Am I wrong in assuming this? If our planets mass were to lessen, would our orbit not change?

Just something I have wondered, again, i am no physics major and have very little physics education.

I think it would be awesome if our space shuttlles caused the earth to fall towards the sun.

Ok, it's been a while:

1) Not everything in the universe is set on an orbital track. The Galaxies are independently moving away from a central point in the universe (I read).

2) I don't recall mass (decreasing or otherwise) having an effect on orbits. Once you're in an orbit you pretty much stay there until you choose otherwise. I may be wrong here, but I don't have the time to research/go through my old forgotten physics notes.

3) The mass 'loss' will be many many orders of magnitude lower than the mass of the planet, so even if I am wrong then the change would be negligible.

The Earth can be the size of a grain of salt and still maintain orbit. Asteroids from city-sizes to pebbles maintain a relative orbit around the sun. If the Earth gained/losted a considerable amount of its mass, the moon's orbit would be affected. Earth would have more/less gravity.

F[sub]g[/sub] = G(m[sub]1[/sub]m[sub]2[/sub])/d[sup]2[/sup]

I am far from an expert here, but wouldnt our planet actually be slowly gaining mass over time from things like space dust, meteorites, and biological growth?

Gravity on the scale of things is a very very very very weak force, compared to electromagnetic for example.

The earth's orbit around the sun will change ever so slightly as we shoot things off into space, but bare in mind the mass of the earth, 5.9742 × 10^24 kg and the mass of the things we send out into space, which are only a very tiny proportion of earth's mass as martyn said

Also, earth is constantly being hit by stuff from space so the actual change in mass of the earth might be closer to zero if everything is evened out.

Also, if the earth did lose mass, we wouldn't fall into the Sun but instead move further.

Biological growth won't change the mass. It's just taking mass from one thing (the ground) and changing it into another form of the same amount of mass.

"Matter cannot be created nor destroyed".

Actually a very small amount of mass will be gained during photosynthesis.

How is that? Photosynthesis is just a process by which energy from the sun is used to create chemical energy from other matter in plant life.

No, Emon's right. Because energy and matter are equivalent, when light energy from the sun is stuck into C-H bonds by plants (or whatever it is that they do), it actually slightly increases the mass of that molecule.

But as far as the orbit goes, I think that our mass actually doesn't make a difference. The strength of the gravitational force on the earth (from the sun) is directly proportional to the earth's mass (by newton's law of gravitation), but the acceleration due to that force is inversely proportional to it (newton's 2nd law), so the mass cancels out. That is:

F(grav) = k (some constant) * M(earth)

F(grav) = M(earth) * a(cceleration) ==> a = F/M

a = kM / M = k, which is just some constant. (This is the same reason for the whole gallileo ~ pisa thing, neglecting air resistance.)


Of course, that's the classical/Newtonian way of interpreting things. I haven't got the slightest idea how general relativity works, so I can't say anything on that front.

JediGandalf nailed this one. Since orbits are dependent only on tangential velocity (V), centripetal acceleration and radius (d), and F=MA, the mass of the orbiting body cancels to give you something like

G(m2)/d[sup]2[/sup]=V[sup]2[/sup]/d

(check my math)

What would bork us would be the sun gaining/losing alot of mass. And before anyone says it, it would be really hard to change the earth's effect on the sun by our gaining/losing mass because you would have to eject matter from/capture matter in solar orbit, which is hard to do from little old earth.

But even with the space program, we haven't actually changed our mass that much, since most of it is still in earth orbit, and can effectively still be considered part of the earth system.

And Emon, nobody counts C[sub]14[/sub]. It decays back into space anyway.:ninja:

Negative on that. While energy and mass may be interconverted, that isn't what goes on in photosynthesis. The energy is just stored as chemical potential, and remains massless. What does happen in photosynthesis is C14 (generated high in the atmosphere by nuclear physics with really high energy EM and whatnot) gets fixed down on earth, where a decay (beta particle) is less likely to just shoot off into space (it'll hit something else), and takes longer to to escape. But it still gets out.

If every lunatic who jumped (and organized 'jump days') caused the earth's orbit to change, Earth would have been sent careening into the sun long ago.

I love physics

Actually that's a bad analogy, because people fall back to the Earth (rather, they are mutually attracted to each other). Rockets don't do that (well, some do, eventually).

i read on wikipedia that the earth's center of gravity is tubgirl's ***

In terms of Earth's mechanical energy in orbit (assuming circular, which it isn't), the energy should be constant and equal to (GMm)/(2r). Therefore if m (earth mass) decreases, r would also have to decrease to maintain the same energy.

The Earth does get a lot of mass from space, I want to say in tons per year, but the mass of the earth is 5.98x10^24 kg so any slight change due to humans launching things into space is effectively meaningless.

Or have we? If you just count the planet, then sure, but if you then expand your view to include objects in orbit, the mass has not changed at all. If you treat this as one closed system (which it is) the orbit around the sun wouldn't be affected.

As described by someone else meteorites would have more of an impact (pun not intended) since they are entering this system from outside it.

Workless space elevator FTW!!!

There's still a few probes like Voyager, and the Mars missions. But of course none of it matters.

it matters it just doesn't matter much at all.

Semievil: No. Photosynthesis involves an increase of net energy within the system which means the mass of the system increases. Energy has mass. The actual amount of energy is infinitesimal which is why it's not mentioned in high school chemistry and biology classes. If you've taken college level physics courses then you should commit suicide for this slight against the gods.

Jumping up and down can't change the earth's orbit because the force of a person jumping and the force of a person landing / attracting the earth is exactly the same.

Yeah... but when the mass decreases, the energy decreases with it. There's nothing saying that the Earth's mechanical energy has to stay constant, only its velocity (which does). [Edit: do you mean potential energy? Same reply, in either case.]


Really, this is exactly the same case as the gallileo-pisa thing. If you drop two equally shaped objects from the same height, but one has a different mass, they hit the ground at the same time, because the term for their own mass cancels out in the equation of motion. The case is EXACTLY the same for orbits, because orbiting is falling, but with enough forward motion that you never hit the ground.

Change in the Earth's mass won't affect it's orbit.

I learneded!

Yes it will, because Earth exerts an attractive force on the sun as well.

The generalization that all objects fall at the same speed is only correct at a terrestrial scale - that is, relatively human-sized similarly-massed objects at more-or-less ground level and only on Earth.

Look up Newton's law of universal gravitation for details you will understand.

Look up general relativity for details you will not.

Jon people here are speaking colloquially stop trying to look real smart.

Think of it like this. Grab a heavy rock and start twirling. You'll notice that you can't stay in the center. You exert a force on the rock and the rock exerts a force on you. This is the same type of effect that the Earth (and other planets) exert on the sun. The sun is not always stationary.

That should be total energy - kinetic plus potential. I'd have to double check to be sure though.

hey guys wait a minute

we forgot to factor in the fact that Rochester, NY is currently the center of mass of the Earth

barycenters ftw

cause thats where mb is lol

I smiled irl.

I saw it coming, and it was still funny.

isn't the moon getting farther away from the earth as time passes?

yes, because the moon is robbing earth of angular momentum. the days are getting longer as a result.

by the time this becomes a problem we'll either be extinct or we'll have the technology to move the moon back into a convenient orbit.

I just love how hes on the bed and like "Crazy women! Don't... hurt me!"

Scientific American had an article recently that said the Earth gets hit by 16.5 metric tonnes per day of meteorites.