[Guest]

Okay, a question just popped in my head. If something completely smooth was traveling on a completely smooth planet, and there was no friction between the two, would the gravity from the planet eventually stop the movement of the other object? I don't know where the question came from, but I was just wondering if anyone knew the answer. And if that is the case, I could make a very nice puzzle out of this

Edited July 21, 2010 by NickFleming

[Guest]

By "perfectly smooth planet" I'm assuming you mean that there are no hills or valleys. Without a change in elevation, the force of gravity on the planet is always orthogonal (at 90 degrees) to the momentum of the object. Which means that the object's momentum would remain unchanged by gravity. Only something pushing on the object in the same non-Euclidean plane* could make it stop.

*Euclidean geometry focuses on dealing with flat planes and lines. Non-Euclidean geometry deals with spheres as "planes," which is what I mean in this case. Only something acting in a direction orthogonal to the sphere's radius could change the momentum of the object on the planet.

Edit: Actually, that's my intuitive answer. I could be misremembering my physics, but that's how I remember it, though since we always treat a planet as if it were a Euclidean plane, I could be wrong on that point. Since there is angular momentum involved and I don't quite remember how that effects the situation.

Edited July 21, 2010 by dawh

[Guest]

No, the gravity of the planet won't stop it. However, other things might.

[Guest]

By perfectly smooth I just meant there would be nothing to cause friction. But yes, flat too. So gravity would not pull it towards the center of the planet, thus slowing down and eventually stopping the object moving?

[unreality]

the net acceleration of an object (which determines its motion) depends on its net force, which requires a nonzero vector component. If it's at a right angle then all the gravitational force is downward orthogonal and I don't believe it would stop its forward motion, just keep it down on the surface. I could be wrong though

[Guest]

I'm gonna need a physics expert lol. You know they recently found out what causes gravity?

[gvg]

Isn't it a particle like a graviton or something?

[Guest]

Isn't it a particle like a graviton or something?

Last I heard, gravitons were theorized, but no one had been able to detect them, so I'm curious as to what NickFleming is referring. Of the four fundamental forces (Weak, Strong, Electromagnetic and Gravitational), only the particles for gravity are missing. Weak has bosons, Strong has gluons and the Electromagnetic force has photons. So the particles theorized for gravity have been called gravitons. If they've found some that would be quite a find.

[Framm 18]

He might be refering to this guy, Erik Verlinde. I can't find the articule that I read but it is something along the lines of gravity is a byproduct of something even larger.

[Guest]

No this is just a theoretical question that popped in my head. I think the answer would be no,it would keep moving, for two reasons: A thats why the moon is still up there in space, a NEARLY firctionless environment (Although technically moon will eventually hit the earth if it doesnt gain mass, but thats by no means an immediate problem) And, since a perfectly smooth planet's surface would be curved, gravity would actaully be pulling it VERY SLIGHTLY in the direction it wanted to go. Now, let me rephrase the question simpler. Now, imagine surface was not curved, and world was flat. Would the force of gravity, pulling perfectly perpendicular to where the object was going, slow and eventually stop the object? You know, tehy recently discovered what causes gravity. Some students at MIT discovered a 'flaw' in an atom that causes the force of gravity. I'll open new "others" topic about this soon.

Edited July 22, 2010 by NickFleming

[Guest]

And, since a perfectly smooth planet's surface would be curved, gravity would actaully be pulling it VERY SLIGHTLY in the direction it wanted to go.

No. If that were true, its speed would increase.

Now, imagine surface was not curved, and world was flat. Would the force of gravity, pulling perfectly perpendicular to where the object was going, slow and eventually stop the object?

No. Force in the direction of motion causes a change in speed, force perpendicular to motion causes a change in direction. Unless opposed, of course, as is the case here. So, nothing happens. Forever.

Although technically moon will eventually hit the earth if it doesnt gain mass

?!?!

[Guest]

?!?!

I think the moon thing is actually true. It's not as bad as Triton going around Neptune, but I've heard that our moon's orbit is not completely stable. It won't change in any significant way on its own for millions of years, but I had heard that the moon's orbit is in very, very, very slow decay. Though I could be wrong, since it's been a long time since I remember hearing about it.

[Guest]

[T]he moon's orbit is in very, very, very slow decay.

Sure. Every orbit is in decay, but... how is adding mass to either body going to stop it?

[Guest]

Sure. Every orbit is in decay, but... how is adding mass to either body going to stop it?

It would change the angular momentum, though just saying adding mass would help is probably a bit simplistic. If mass were altered between the two bodies in just the right way, it might be possible to completely stabilize the orbit, but of course since this is the real world and not some physics exercise, there would undoubtedly be other complications even if the masses could be easily altered.

[Guest]

It would change the angular momentum, though just saying adding mass would help is probably a bit simplistic. If mass were altered between the two bodies in just the right way, it might be possible to completely stabilize the orbit, but of course since this is the real world and not some physics exercise, there would undoubtedly be other complications even if the masses could be easily altered.

Er... no. The Moon's orbit will eventually decay (it is currently increasing in altitude, but that won't last forever) because of orbit perturbations, which are not going to go away just because you added/subtracted some mass, which you could do with or without changing the angular momentum of the system.

[Quag]

perhaps he meant add velocity, or subtract velocity, depending on what ya want the moon to do. however this thread was about a ball on perfectly spherical world with no form of friction. an impossible state, i know but theoretically the ball would never stop.

[Izzy]

Umm, what? Last I learned the moon and Earth are actually separating at a rate of around 4 cm/year.

I'd tell you why, but I blew off that class, crammed all night before the exam, managed to get a 100, and ended up with like an 89.8 A. >_>

If I remember correctly (meaning, "don't quote me on this "), it's because of the distorted shape the moons gives the earth (spring tide, neap tide, tides in general). Since the Earth rotates faster, the distorted shape pulls the moon along with it, but the moon pulls the other way. So, Earth speeds up moon, moon slows down Earth. That's called Tidal Friction, and Earth's energy is transfered to the moon's orbit, increasing its size.

..Or. something like that.

*edit* ..I didn't actually /read/ the thread. I just.. saw moon stuff.

I think that holds true regardless of mass, so long the moon rotates slower.

..Which.. actually made me think of another question. If the moon rotated faster than the Earth, would its orbit size decrease?

Edited July 23, 2010 by Izzy

[Guest]

Umm, what? Last I learned the moon and Earth are actually separating at a rate of around 4 cm/year.

Yes, but that won't last forever.

If I remember correctly (meaning, "don't quote me on this "), it's because of the distorted shape the moons gives the earth (spring tide, neap tide, tides in general). Since the Earth rotates faster, the distorted shape pulls the moon along with it, but the moon pulls the other way. So, Earth speeds up moon, moon slows down Earth. That's called Tidal Friction, and Earth's energy is transfered to the moon's orbit, increasing its size.

Almost. It's actually not the shape of the moon, but the fact that Earth is rotating about its axis faster than the Moon orbits about the earth (a month is longer than a day). So, the tide sort of sloshes ahead of the moon, pulling it along and increasing its orbit, and simultaneously slowing down Earth's rotation. Eventually, a month will last a day, and at that point, it will start to decay.

..Which.. actually made me think of another question. If the moon rotated faster than the Earth, would its orbit size decrease?

Yup.

[Izzy]

...And if the moon is speeding up and the Earth is slowing down. Oh. That's not a very favorable future.

[Guest]

Jut going by what my dad told me, but if the moon GAINS mass, it will actually start going farther away from teh earth, instead of closer. I fear that Rookie may consider this off-topic and he'll message ma gain... Sorry Rookie!

[Guest]

gravity will eventually cause contact between the 2 objects, unless the moving object is hovering.