Thread: earth and moon

We all know that the earth has more attraction power than the moon.Then why don't the earth attracts the moon and the moon collides with the earth?

Originally Posted by sciman

We all know that the earth has more attraction power than the moon.Then why don't the earth attracts the moon and the moon collides with the earth?

Damn I knew this one but I cant quite remember. The moon is sort of falling towards the earth but never actually gets there due to the curvature of the earth, it's like the moon is constantly falling towards the earth only for the earth to make itself the original distance away( a bit like running on a treadmill)............I think thats about the size of it.


Do you know a bit of general relativity?

Originally Posted by GhostofMaxwell

Do you know a bit of general relativity?

what's wrong with explaining it classically?

the earths gravity provides a centripetal force which keeps the moon at a particular radius from the earth based on it's angular velocity. whether or not my memories of last years highschool physics is acurate is a side issue however as i don't see why we need to introduce a concept such a general relativity.

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Originally Posted by wallaby

Originally Posted by GhostofMaxwell

Do you know a bit of general relativity?

what's wrong with explaining it classically?

the earths gravity provides a centripetal force which keeps the moon at a particular radius from the earth based on it's angular velocity. whether or not my memories of last years highschool physics is acurate is a side issue however as i don't see why we need to introduce a concept such a general relativity.

Its based on the angular velocity ? My memory is its curvature.

Originally Posted by GhostofMaxwell

Its based on the angular velocity ? My memory is its curvature.

But then wouldn't an analogous question be, why doesn't the moon slide all the way down this curve and smash into the Earth?

No because I thought the curved shape itself was what was stopping the moon ever getting to the surface, although it is hurtling towards said surface.

but doesn't that imply two opposite kinds of curvature acting at once?

even back in the day, when i could actually be bothered with reading about relativity, i was under the impression that the moons velocity through the curved spacetime created by the earths mass kept it in its orbit. nowdays i wonder whether explaining a simple phenomenon in terms of technical university level theories.

by the way does anyone know if relativity is under graduate or post graduate university work?

Originally Posted by wallaby

by the way does anyone know if relativity is under graduate or post graduate university work?

Definitely undergraduate. But it may be involved in postgraduate depending on what you are specializing in, I guess.

Originally Posted by GhostofMaxwell

Definitely undergraduate. But it may be involved in postgraduate depending on what you are specializing in, I guess.

hopefully one of the things i'll get to study in the coming years then.

Have you ever been on the merry-go-rounds they used to have in ordinary parks?

couldn't have put it better myyyyy-self.

Now lets prove that the speed of that merrrrrrryyyyyyy-go-around is caused by thousands of years of mind-over ma-ma-ma-ter that men in-the mooooooooon have inflicted on us-sssssss through lu-lu-lu-nar calendars.

The simple answer to the question is, it's centrifugal force. If you tie a weight to a string and spin it in a circle, the tension in the string keeps it from flying off at a tangent. In the same way, the gravitational attraction keeps the moon from flying off into space at a tangent to its orbit.

Some purists will say you should call it centripetal force, others will talk about relativity and space-time curvature, but the simple answer you are looking for is centrifugal force.

Originally Posted by Harold14370

but the simple answer you are looking for is centrifugal force.

Well I dont know about purists , but centrifugal force doesn't exist because its the change in velocity(angular) of the central object that is pulling on the orbiting object not the orbiting object that is pulling away from the rotating central object.

Originally Posted by Harold14370

The simple answer to the question is, it's centrifugal force. If you tie a weight to a string and spin it in a circle, the tension in the string keeps it from flying off at a tangent. In the same way, the gravitational attraction keeps the moon from flying off into space at a tangent to its orbit.

Some purists will say you should call it centripetal force, others will talk about relativity and space-time curvature, but the simple answer you are looking for is centrifugal force.

i'm afraid the difference between centrepital and centrifugal force goes beyond the name. using the weight attatched to a string idea, as you pointed out that if we were swing this mass about our heads then the tension in the string would act on the mass to keep the mass in it's circular path and we technically call this centripetal force. however what of a reaction force?

as the mass resists changes in it's velocity there will be a reaction force in the string which we call the centrifugal force, this force however does not act on the mass but instead acts on the string.
action: centripetal force
reaction: centrifugal force

See what I told you about the purists. Everybody has felt the tug of the string on their hand when swinging a weight on a string, or the "force" that pushes them to the outside of a car going around a corner. Yes, it's a fictitious force resulting from the choice of a rotating frame of reference. Will that help sciman at this point in time? Maybe after he has taken some physics courses and knows what you mean by an inertial frame of reference.

Einstein tells us that gravity is a fictitious force too, and we are really feeling a warp in space time. I don't know about you, but that doesn't help my understanding much, and I still think of gravity as a force.

Originally Posted by Harold14370

See what I told you about the purists.

and just what is that meant to mean, i reguard centrifugal and centrepital force to be two things and you never refuted that.

Originally Posted by Harold14370

Will that help sciman at this point in time? Maybe after he has taken some physics courses and knows what you mean by an inertial frame of reference.

so why are you refering him to centrifugal force which uses these concepts? if sciman were to now go google centrifugal force there's going to be information of inertial frames in those links, investigation reveals that sciman would also be shown links to pages on centripetal force as well. so how does this help sciman?

Originally Posted by Harold14370

Einstein tells us that gravity is a fictitious force too, and we are really feeling a warp in space time. I don't know about you, but that doesn't help my understanding much, and I still think of gravity as a force.

which means that you should study forces before fictitious forces doesn't it?

which means that you should study forces before fictitious forces doesn't it?

No, I think most people study Newtonian mechanics before relativity. I find it a lot easier to remember things I can relate intuitively to everyday experience than the more abstract mathematical concepts.

Mathematics being the victor yet again?

Could space-time be derived by purely theretical means as opposed to observed cause and effect?

Mmmmmmmmm.

Originally Posted by Harold14370

No, I think most people study Newtonian mechanics before relativity. I find it a lot easier to remember things I can relate intuitively to everyday experience than the more abstract mathematical concepts.

and that would be where our brains differ i suppose.

on review it would appear i failed to properly communicate what i was trying to, one of my prominent faults unfortunately. what i was trying to get across is that, yes relativity attributes the phenomenon associated with gravity to warped spacetime however spacetime being a mathematical object means that it is mathematically a fictitious force. however in the senario of circular motion we have a force pulling on a mass and a mass that doesn't want to budge resulting in the appearance of a force, i personally find it much easier to work with the center seeking force known as centripetal force.

Originally Posted by wallaby

Originally Posted by Harold14370

No, I think most people study Newtonian mechanics before relativity. I find it a lot easier to remember things I can relate intuitively to everyday experience than the more abstract mathematical concepts.

and that would be where our brains differ i suppose.

on review it would appear i failed to properly communicate what i was trying to, one of my prominent faults unfortunately. what i was trying to get across is that, yes relativity attributes the phenomenon associated with gravity to warped spacetime however spacetime being a mathematical object means that it is mathematically a fictitious force. however in the senario of circular motion we have a force pulling on a mass and a mass that doesn't want to budge resulting in the appearance of a force, i personally find it much easier to work with the center seeking force known as centripetal force.

But sciman already knows about the center seeking force - gravity. What he wanted to know is what keeps the moon up.

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