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sundaju
2006-Aug-23, 03:30 PM
What forces are acting on the moon to keep it in the orbit around the Earth?

The well known one is "earth's gravity on moon minus moon's gravity on earth"

hhEb09'1
2006-Aug-23, 04:02 PM
Why minus?

The force is the same.

cjbirch
2006-Aug-23, 04:06 PM
The gravitational force between the Earth-Moon System, which tries to pull the moon towards the former, as it orbits about the Earth is known as Centripetal force. This force is balanced by Centrifugal force, which pulls on the Earth keeping the moon in motion. The balance between Centripetal and Centrifugal force are what keeps the Moon orbiting the Earth.
This reasoning provides an understanding for how the Moon stays in orbit around the Earth. However you could also look at Einstein's Theory of GR to explain why the Moon orbits the Earth in the way it does. GR provides that objects with mass curve the spacetime within their vicinity and it is this curvature which influences the motions of other objects. The greater the objects mass and density, the larger the curvature of spacetime will be. It follows therefore that the Moon orbits the Earth because of the Earth's curvature of spacetime within the vicinity of the Moon. This relationship between mass and curvature cause the gravitational and Centripetal forces to exist, causing the Moon to orbit Earth.

CJ

sundaju
2006-Aug-23, 04:07 PM
Why minus?


Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so?

hhEb09'1
2006-Aug-23, 04:38 PM
Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so?And they've equal magnitude. When you subtract...well, it depends on how you do the subtraction. You can get zero, or twice the quantity.

adamsoltan
2006-Aug-23, 05:42 PM
Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so?

That's the force that the Earth 'feels'. The original question was about forces acting on the Moon and those are, as already mentioned, the centripetal and the centrifugal forces. From the mechanics point of view it's the same as if you would start spinning around holding a string with a rock attached to the other end. The only differance is that with gravity you don't see the agent keeping the two objects together. Hope this clarifies a bit.

/Adam

pghnative
2006-Aug-23, 05:57 PM
Keep in mind that the Newtonian way of looking at this is that there is only one force involved: gravity.

Of course, force = mass times acceleration. So the single force (gravity) is accelerating the moon*.

The acceleration of the moon does not change its speed; instead, the acceleration changes the direction of the moon's travel. Thus the moon travels in an approximate circle around the earth instead of flying off.

Good thing too, or we wouldn't have all of these Moon Hoax threads around here...

*Of course, gravity is accelerating the Earth toward the moon, also, but we're just talking about the moon so far in this thread.

Blue Fire
2006-Aug-23, 05:57 PM
But within the concept of curved space due to mass, the moon would not be "held" in orbit around the Earth by any force of "attraction" but would instead be following a "straight line" in curved spacetime. Like a marble that has been sent rolling around a canonball resting on a trampoline. The canonball deforms the trampoline like the Earth deforms spacetime. And so the marble is just following the curvature of the trampoline with no force of attraction between the marble and the cannonball. Right?

grav
2006-Aug-24, 12:37 AM
But within the concept of curved space due to mass, the moon would not be "held" in orbit around the Earth by any force of "attraction" but would instead be following a "straight line" in curved spacetime. Like a marble that has been sent rolling around a canonball resting on a trampoline. The canonball deforms the trampoline like the Earth deforms spacetime. And so the marble is just following the curvature of the trampoline with no force of attraction between the marble and the cannonball. Right?But could this be demonstrated with an analogy that doesn't originally require gravity (or other similar force) already acting on the cannonball on the trampoline in order to create the "curved spacetime" in the first place?

hhEb09'1
2006-Aug-24, 02:00 AM
But could this be demonstrated with an analogy that doesn't originally require gravity (or other similar force) already acting on the cannonball on the trampoline in order to create the "curved spacetime" in the first place?I think you're asking for an example, rather than an analogy, maybe?

The point of the analogy is that the marble is not really attracted to the cannonball by the cannonball (at least, to the extent that it is), but is a result of the configuration of the path. But what you're modeling is the curvature of spacetime, and it's pretty hard to model curved time without actually doing it. :)

Jens
2006-Aug-24, 03:30 AM
Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so?

It sounds from your understanding that two masses of the same mass wouldn't orbit one another, because the force would amount to zero. But clearly that isn't true. In actuality, the earth is pulled to the moon with the same force that the moon is pulled to the earth. It's just that the earth is so much more massive that it doesn't appear to move. But it does.

PhantomWolf
2006-Aug-24, 03:43 AM
The gravitational force between the Earth-Moon System, which tries to pull the moon towards the former, as it orbits about the Earth is known as Centripetal force. This force is balanced by Centrifugal force, which pulls on the Earth keeping the moon in motion. The balance between Centripetal and Centrifugal force are what keeps the Moon orbiting the Earth.

Ummmm.... no.

Centrifugal force doesn't exist. It is a mathematical result and in only found inside an accelerating frame of referance. ie, if you are sitting in a car as it rounds a corner, you experinece a "force" on you making you press up against the door of the car. In reality though the force is being applied to the car making it turn (accelerate towards the centre of the bend), you are attempting to travel in a straight line and the car door intercepts you and applies a force on you to change your direction (accelerate you) to match its own new velocity. (remember acceleration is a change in velocity, and velocity has two components, magnitude (speed) and direction. A change of direction without a change in magnitude is still an acceleration.)

Newton noted that "Any body set in motion will continue in that motion untill such time another foce acts upon it."

If we apply this to the moon, its forward motion is unchanged as it orbits (the magnitude portion of its velocity does change due to Kepler's Laws of Orbital Motion and rotational inertia.) It just wants to travel in a straight line at a constant speed, and would do so if there was no other force acting on it. Now we add Gravity. This will accelerate the moon directly towards the centre of mass of the Earth-Moon system, in other words, the gravity well acts like a force attempting to push (or pull) the moon to the centre of mass. However this is always occuring at an angle to the direction of motion of the moon, so what it does is change the veleocity of the moon by changing its direction rather than its magnitude (the magnitude will change slightly throughout the orbit due to the moon's orbit not being a perfect circle.)

This shows us that no other force is required to keep it in orbit, just that applied by gravity. There is no "balancing of Centipedal and Centifugal forces keeping the moon in position," there is just gravity acting on an object in motion by drawing the moon towards a centre point, and the moon missing it because it is moving forwards too fast.

hhEb09'1
2006-Aug-24, 03:59 AM
This shows us that no other force is required to keep it in orbit, just that applied by gravity. There is no "balancing of Centipedal and Centifugal forces keeping the moon in position," there is just gravity acting on an object in motion by drawing the moon towards a centre point, and the moon missing it because it is moving forwards too fast.cjbirch was just looking at it from a different point of view, with regards to the so-called inertial forces.

If you push on an object, does it push back? The force that is felt on your hand is the inertial force, of the resisting object. The tendency of an object to move in a straight line is inertia, it will resist a force that tries to move it from that straight line with an inertial force. OTOH, it is very easy to get confused. :)

Blue Fire
2006-Aug-24, 05:24 AM
I think you're asking for an example, rather than an analogy, maybe?

The point of the analogy is that the marble is not really attracted to the cannonball by the cannonball (at least, to the extent that it is), but is a result of the configuration of the path. But what you're modeling is the curvature of spacetime, and it's pretty hard to model curved time without actually doing it. :)
Thanks for your response in my stead! I was unable to get back to this thread until now. And, I doubt I could have responded any better than you did anyway.

cjbirch
2006-Aug-24, 11:23 AM
This shows us that no other force is required to keep it in orbit, just that applied by gravity. There is no "balancing of Centipedal and Centifugal forces keeping the moon in position," there is just gravity acting on an object in motion by drawing the moon towards a centre point, and the moon missing it because it is moving forwards too fast.

Of course, it was just a different perspective for the same scenario! The moon is falling towards the Earth, just like if we dropped a stone onto the surface, and then we get to again why does the Moon travel around the Earth in an orbit rather than simply fallling onto it? Again it is true that if there were no gravity present the moon would travel in a straight line, at the same time the Moon's path along this straght line would cause it to move away from the Earth.
Therefore without taking into account GR and the curvature of spacetime, we could simply assert that the amount the Moon falls towards the Earth due to gravity is enough to offset the straight-line path it would take if gravity were not acting in order to deflect it. This is the balance I was referring too, simply using a different point of view.
All being said we actually arrive at a position however that affirms that the Moon is in actuality, perpetually falling around the Earth.

CJ