Quick gravity question

[phillip1882]

okay my father and i were disscucing gravity and we really couldn't come to a consenus as to wheter or not there's an equal and oppsite side to gravity, for example consider the moon in orbit around the earth, gravity pulls inward on the moon but does the moon have a pull outward in resistence to that force? i hypothised that no, there is no out ward pull, otherwise the moon would rapidly orbit at a futher and futher distance. my father was on the side of yes, as every force has an oppisite effect, for example with a rock swung on a string, there is an outward pull.

[coldcreation]

Good question.

 

That's why gravity is called the 'odd-ball' force.

 

:hyper:

[Moontanman]

okay my father and i were disscucing gravity and we really couldn't come to a consenus as to wheter or not there's an equal and oppsite side to gravity, for example consider the moon in orbit around the earth, gravity pulls inward on the moon but does the moon have a pull outward in resistence to that force? i hypothised that no, there is no out ward pull, otherwise the moon would rapidly orbit at a futher and futher distance. my father was on the side of yes, as every force has an oppisite effect, for example with a rock swung on a string, there is an outward pull.

 

Yes there is an outward force on the moon, it's called centrifugal force, just like the rock on a string. the moon also pulls the earth toward it, IE the tides. but no there is no opposite force to gravity, gravity is only positive, no anti gravity.

[phillip1882]

well one topic is genneral realtivity, and the other is baisc classical physics. also i wnated to seperate the two because i felt they were both good questions/observations.

[coldcreation]

well one topic is genneral realtivity, and the other is baisc classical physics. also i wnated to seperate the two because i felt they were both good questions/observations.

 

If one drops an apple, why does it drop to the ground? Most everyone will respond by saying that the Earth’s gravitational force attracts the apple, and so it falls to the ground. The persons vaguely familiar with GR might answer that the Earth creates a field around it, and like a bowling ball on a rubber sheet, the Earth deforms, ‘stretches,’ the fabric of space causing objects to fall into the Earth’s gravitational potential well. This answer is identical to the first response in that the Earth’s field results in objects being attracted to the Earth. Gravity can accordingly and alternatively be described in at least two ways, both of which are equally justified:

 

(1) An apple is pulled towards the Earth.

(2) An apple is pushed towards the Earth.

 

One might argue that it makes no difference. The observation is the same. It’s the same force. Yes true, and the lines of force are pointing in the same direction, but the source is not the same. :rolleyes:

 

 

CC

[Little Bang]

There is a force acting on the moon phillip. The tides actually speed the moon up slightly, hence the moon moves away from the Earth about 1.75 inches every year.

[modest]

okay my father and i were disscucing gravity and we really couldn't come to a consenus as to wheter or not there's an equal and oppsite side to gravity, for example consider the moon in orbit around the earth, gravity pulls inward on the moon but does the moon have a pull outward in resistence to that force? i hypothised that no, there is no out ward pull, otherwise the moon would rapidly orbit at a futher and futher distance. my father was on the side of yes, as every force has an oppisite effect, for example with a rock swung on a string, there is an outward pull.

 

If you think of gravity as a force (as others have pointed out, this isn't exactly correct) then yes, there is an equal and opposite force.

 

 

Here F1 is equal and opposite to F2 either of which can be calculated with newton's equation of gravity there. The force pulling the moon to the earth is equal and opposite the force pulling the earth to the moon.

 

~modest

[phillip1882]

i realize the moon pulls on the earth with as much force as the earth pulls on the moon. my question deals more with centralfugal force and whether it exists with gravity. for example, when you simply drop a rock, there is no centralfugal force, or at least i woudn't think there to be, so why would there be centralfugal force with an orbiting body? there is only left to right motion and the constant pull. or at least that's what i would think.

[KALSTER]

Remember that the moon is basically in freefall. That means there is a constant force between the earth and the moon, as has been described. The moon also has a motion perpendicular to the direction of gravitation. This perpendicular motion is enough for the moon to fall in a near perfect circle around the earth, constantly accelerating (by virtue of constantly changing vector direction) and thus creating a force in the opposite and near equal direction of gravitation, keeping the orbit near stable. I suspect you already know all this and I have been wasting my time.:naughty:

 

So whether gravity is purely geometrical or whether it is mediated by a graviton (whose anti-particle would also have positive gravity as result by the way), gravity is always attractive. Centrifugal force and gravity are seperate entities and do not co-exist in any mutual fashion.

[freeztar]

i realize the moon pulls on the earth with as much force as the earth pulls on the moon. my question deals more with centralfugal force and whether it exists with gravity. for example, when you simply drop a rock, there is no centralfugal force, or at least i woudn't think there to be, so why would there be centralfugal force with an orbiting body? there is only left to right motion and the constant pull. or at least that's what i would think.

 

Good questions Phillip!

 

It's important to note that centrifugal force is often thought of as a pseudo-force, compared to gravity which is a real force.

 

See if this helps you:

Centrifugal force - Wikipedia, the free encyclopedia

 

Also, read the intro to this article.

[modest]

i realize the moon pulls on the earth with as much force as the earth pulls on the moon. my question deals more with centralfugal force and whether it exists with gravity.

 

I'm sorry, I misunderstood.

 

Yes, as others have said, for an object to be in orbit, the force of gravity will be equal to the centrifugal force.

 

The force of gravity can be calculated:

[math]F_G=\frac{GM_1M_2}{r^2} [/math]

and the centrifugal force can be calculated:

[math]F_C=\frac{Mv^2}{r} [/math]

The force of gravity will equal the centrifugal force:

[math]F_G=F_C [/math]

or,

[math]\frac{GM_1M_2}{r^2}=\frac{Mv^2}{r} [/math]

It might be a fun mental exercise to plug in the numbers which you can get here:

Moon Fact Sheet

and see if this relationship works :)

 

for example, when you simply drop a rock, there is no centralfugal force, or at least i woudn't think there to be, so why would there be centralfugal force with an orbiting body?

 

Using the equation above, you could calculate the centrifugal force of a rock sitting on the equator. It is spinning around because the earth is spinning - so there is a force associated with a rotating reference frame on the rock. In other words, it is being spun outwards.

 

However, a rock on the surface is not in orbit. The earth rotates about a thousand miles per hour at the equator which is about a half a kilometer per second. To be in orbit it would need to go 11.2 kilometers per second. Something in orbit then has greater centrifugal force than something sitting on the surface because it has a greater speed. For the rock on the surface the centrifugal force is much less than gravity, so it falls. For a rocket in orbit or the moon in orbit, the centrifugal force is equal to gravity.

 

Here is a website with more info and a better explanation than mine:

 

What is centrifugal force?

 

~modest

[phillip1882]

...and thus creating a force in the opposite and near equal direction of gravitation, keeping the orbit near stable.

yes this is what im after. though are you certain the force is oppisite and equal to gravity? i would think the force of motion would be perpendicular to gravity. like i said it's unclear to me. it seems to me if there was truly a force oppisite and equal to gravity, then there would be no gravity. of course i'm no physist.

[KALSTER]

yes this is what im after. though are you certain the force is opposite and equal to gravity? i would think the force of motion would be perpendicular to gravity. like i said it's unclear to me. it seems to me if there was truly a force opposite and equal to gravity, then there would be no gravity. of course i'm no physist.

Yes, the centrifugal force is equal and opposite in the earth-moon system. It is a result of the angular velocity of the body and the force that provides the tension in the rope when you swing a rock tied to a rope in a circular motion. The centrifugal force is the force that provides pseudo-gravity in fictitious rotating space stations. If the force had not been equal, the moon would have fallen onto the earth or it would have drifted off into space.

[Natural]

yes this is what im after. though are you certain the force is oppisite and equal to gravity? i would think the force of motion would be perpendicular to gravity. like i said it's unclear to me. it seems to me if there was truly a force oppisite and equal to gravity, then there would be no gravity. of course i'm no physist.

 

I think much of the problem with answering your question is that everyone is assuming that the quantum function of gravity has been explained by modern science. Which it has not. (theories yes... proof no)

In fact there is a whole other set of explanations for the effects of gravity that assumes that gravity is a pushing force instead of a pulling force and that the reason that objects seem to be drawn towards each other is because each object moves in a line which corresponds with the shadowing effect between two objects. In other words each object is "pushed" by gravitons hitting them from all sides except the areas facing each other making one side the "back" (the back being the side opposite the corresponding object), thus pushing them toward each other because of the lack of push on the "front". (in the shadow)

So if you think of something being hit from all sides there would definitely be an "equal and opposite" effect.

Of course this might be thought of as blasphemy by the members of the "Church of Relativity".:hihi: And if everyone can keep their panties from getting in a twist over that thought, let me reiterate that this is just another theory.

But my point is that there are other theories that work just as well as GR to explain gravity. (some even better and more logical)

Because as I have said before... the problem with the bowling ball on a rubber sheet analogy is that a sheet is 2D and space is 3D. Thus no "surface" to bend. And don't get me started on the fact that space doesn't even actually exist.

:(:hihi:

[modest]

I think much of the problem with answering your question is that everyone is assuming that the quantum function of gravity has been explained by modern science. Which it has not.

 

1. Phillip's question is perfectly answerable with any theory of gravity. Notice I answered his question perfectly well using Newtonian gravity and the answer would hold true with GR or loop quantum gravity. The answer to his question has nothing at all to do with different theories of gravity, but simple and measurable effects of gravity.
   
2. No one (not in this thread or in the scientific community) is "assuming that the quantum function of gravity has been explained". That is just plain wrong.
   

 

The topic of the rest of your post has recently been discussed in 15394 which you might enjoy.

 

~modest

[Natural]

1. Phillip's question is perfectly answerable with any theory of gravity. Notice I answered his question perfectly well using Newtonian gravity and the answer would hold true with GR or loop quantum gravity. The answer to his question has nothing at all to do with different theories of gravity, but simple and measurable effects of gravity.
   
2. No one (not in this thread or in the scientific community) is "assuming that the quantum function of gravity has been explained". That is just plain wrong.
   

 

The topic of the rest of your post has recently been discussed in 15394 which you might enjoy.

 

~modest

 

Right. I wasn't trying to be rude just extremely factual. I was just saying that a real "answer" would be assuming that the answerer had all the facts. And that the facts didn't rely on one of multiple theories. Which would make any answer just another theory and not really an answer.

It's all just a matter of semantics I guess.

:(

[GAHD]

okay my father and i were disscucing...the moon in orbit around the earth, gravity pulls inward on the moon but does the moon have a pull outward in resistence to that force? i hypothised that no, there is no out ward pull, otherwise the moon would rapidly orbit at a futher and futher distance. my father was on the side of yes, as every force has an oppisite effect, for example with a rock swung on a string, there is an outward pull.

Both observations are right and wrong depending on how you yourself perceive gravity and space.

 

i realize the moon pulls on the earth with as much force as the earth pulls on the moon. my question deals more with centralfugal force and whether it exists with gravity...why would there be centralfugal force with an orbiting body?

When dealing with orbital mechanics, there's a few weird ideas. Eg: the moon is orbiting in a straight line that is also a circle. That seems counter intuitive at first but when you think of space as topography, the moon is only moving one direction; forward. Space itself is warped by the earth and moon so that "straight" suddenly becomes relative to a 'Vally' formed by the interference of the two, and the moon just happens to be exactly far enough out that it's Inertia relative to the Vally is enough that it never actually "falls"

 

The same is also true of the earth, the earth's orbit around the sun looks more like a sign wave than a circle or oval when plotted with the moon in consideration; and the earth is also traveling "straight spiral" around the sun.

 

Some would explain these lines as Geodesics.

 

You can use the string in your earlier bola example as an analogy for gravity, but it only goes so far. What might help is if you thought of throwing a rock and how it bends towards the ground; throw it fast enough and by the time it tries to hit the ground, the ground has curved away from it and it just keeps curving...

 

There is an old mac classic program I had about 15 years ago for orbital simulations that explained things quite well, I'll have a look to see if I can find a new version...no promises though.