Question about gravity.

Originally posted by spy66
Ok since you bring this up.

I have a question for you. In this image you have three masses made of the same materials within a vacuume space, but they are different in size. My question is. Which mass attracks which? Or do they stay in the same positions?

They would attract each other, all moving somewhere into the centre.

We are still waiting for you to explain why gravity does not work in a vacuum, or how satellites work in a vacuum.

Originally posted by spy66
In other words there is no gravity reduction towards center. >A vacuume have no gravity.

Objects are effected by gravity, being in a vacuum does not matter

So if there is no gravity from N to S how can it fall towards the center?

Why claim there is no gravity?

Originally posted by hellobruce

Originally posted by spy66
In other words there is no gravity reduction towards center. >A vacuume have no gravity.

Objects are effected by gravity, being in a vacuum does not matter

So if there is no gravity from N to S how can it fall towards the center?

Why claim there is no gravity?

A vacuume is a space without gravity. A vacuume is a space with no matter, temprature or time. Look it up.

It dosent matter what is surrounding this tunnel. it wont effect the vacuume.

Originally posted by spy66
A vacuume is a space without gravity.

Wrong. What makes you think a vacuum somehow stops gravity.

A vacuume is a space with no matter, temprature or time. Look it up.

No time in a vacuum? Just where are you getting your silly ideas from?

You have no understanding at all of physics - care to explain how a satellite works in a vacuum then?

You do realize that garvity and magnetism are to separate things?

reply to post by spy66


@spy66 gravity is one of the four fundamental interactions/forces between matter. The interesting property of these interactions is that they require no medium.

In fact if you look a bit closer you will notice that all matter is pretty much empty space. The forces between matter particles is what makes matter look solid.

Earth is in the vacuum of space, yet there is gravity... your vacuum tube would have no effect on gravity, the object would fall towards the gravity source.

If the object was in a vacuum, away from all sources of mass, then it would not fall; it would be suspended in space…and have its own gravity field, just like earth or any other object in space.

Originally posted by spy66

My question is:

If you built a vacuume tunnel that runs from the North pole throught earths center and to the South Pole. And on the North Pole you placed a 10kg weight into the vacuume tunnel. Would the weight fall/travel to the South Pole?

My teacher/Professor tells me that the 10kg weight would fall to the South Pole. I am telling him that the 10kg weight would not travel anywhere.

I have also googled this question and found that a lot of other people agree with my teacher. But they are all wrong. Does anybody here know why my teacher is wrong?

Im baffled
You asked this question on a teacher...and he answerd you like that?!?!?


Listen to SplitInf...
Even i with my limited understanding
accepts the fact that, it would fall,
come somewhere in the middle, maybe
abit further and then reverse...
Vacuum or no vacuum doesent matter...

reply to post by spy66


Spy, you are misunderstanding what a " vacuum" is.
A vacuum is an absence of gaseous phase matter, not an absence of all matter.
Like so many people have said it has no effect on the force of gravity, none what so ever.
And all mass has a gravitational field, very likely down to the quantum level, for those interested look into quantum gravity detection experiment being put together by Cal State Humbolt, fascinating stuff.
Anyhoo, Spy in your OP, the scenario you have works like this,
Your tunnel is a vacuum, the only effect it will have is an absence of media to provide friction, that's it ,it will not effect gravity at all.
So now , the force of gravity follows an inverse square relationship with distance from the center of mass, that is to say the force falls off at a rate that is proportional to the square of the distance between the objects.
That's why the acceleration due to gravity , which at the earths surface is 32/sec^2, falls off so quickly.
The weightless of " space" is not a gravityless environment, it is micro gravity environment, as long as there is mass there is gravity.
With all that in mind, if one were to drop your mass down the tunnel from the north to south pole, your mass would start with a specific amount of gravitation potential energy, it will accelerate at 32 feet/sec^2, to start with. It will change because the of the differing densities of the materials that make up the earth and their relative masses. But to keep it simple we will say it falls at a 32ft/sec^2. As it falls the gravitational potential energy is turned into kinetic energy, according to the equation U=mgh, where U is gravitational potential energy
m is mass, g, acceleration due to gravity 32ft/sec^2 and h is height of object or in this case the distance from the theoretical center of gravity.
Now a body in motion will stay in motion a body a rest will stay at rest, that is verbal expression of conservation of momentum. In absence of external forces acting on a moving body a moving body will stay in motion.
Back to the hole, your mass accelerates toward the centers of gravity, turning gravitational potential energy to kinetic energy. As it it approaches the center the force of gravity in front of it increases, because of the increasing density, but conversely as you pass through the earth all of the mass behind the falling mass has a gravitational effect on the motion of the moving mass slightly slowing it down.
This is where I differ from the poster who said, that in a vacuum, it would be a simple oscillation scenario and the mass would oscillate back and forth forever, it would not. If your are using the.force of gravity as a force in the analysis, you can't also discount it.
Classical newtonian physics treats the earth as a point mass, but if you are looking at the mass of the earth as as a system then you have have to account for the mass in relationship to the position of the object and differing forces of gravity due to changes in density as it moves through the gravitational system that is the earth as a whole. It would appear to follow a simple straight line oscillation back and forth losing energy and eventually settling and coming to rest at the bottom of the gravity well.
No it would follow a extremely eliptic path orbiting the theoretical center of gravity. This orbit would decay over time in to rotational motion of the original mass at the center of the earth.
So the mass would swing back and forth not going quite as far each trip and it would eventually come to rest at the center of the earth, spinning, at a very high rate it might add as momentum must be conserved.
All of the momentum of the original mass has to be maintained it is converted from linear momentum to rotational momentum.
This is why stars and planets rotate around thier own axes and why planets and asteroids and comets rotate around stars and why star systems orbit the center of the galaxy and galaxies orbit even larger groups of each other.

Originally posted by spy66


Totally ignorant guy here, so forgive if this is stupid: Isn't the universe in a vacuum? If so, according to your premise, none of the planets or galaxies would be able to move. Right?

Ok since you bring this up.

I have a question for you. In this image you have three masses made of the same materials within a vacuume space, but they are different in size. My question is. Which mass attracks which? Or do they stay in the same positions?







I would assume that the highest mass attracts the others.

Originally posted by spy66

Originally posted by alfa1
My opinion is that it would accelerate as it falls, passing through the center of the earth and reaching the south pole 42 minutes later. On its way up to the south pole it would be decelerating and reach zero speed at the surface.

Then, like a pendulum, it would return to you at the north pole another 42 minutes later.... then back to the south pole, and so on forever.

Of course this assumes perfect vacuum, no touching the sides of the tunnel, and so forth.

edit on 20-10-2012 by alfa1 because: (no reason given)

Not true.

In a perfect vacuume isolate inside a tunnel from N to S. There is no way the 10kg would be effected by earths gravity. How can it be effected by earths gravity inside the vacuume tunnel?

edit on 27.06.08 by spy66 because: (no reason given)

Gravity works in space, which is a vacuum.

Perhaps you think that gravity is something like an anti-buoyancy force created in the atmosphere, but it isn't. Gravity is caused by the curvature of space-time, which itself is caused by very large masses.

If you seal a vessel (like a glass bell jar) and pump all the air out, then the vessel contains a vacuum. If there is a ball bearing inside that vessel when you pump out the air, the ball bearing does not suddenly float off in its own but remains gravitationally attracted towards the earth.

Time and gravitation both exist in a vacuum.

The 10kg mass will obey the laws of simple harmonic motion while falling but the Earth itself is axially tilted and in orbit around the Sun, this would cause the falling mass to keep bumping against the sides of the tube and would cause the potential energy to deplete, eventually the mass would come to rest at the center of mass of the Earth but this would likely take a long time.

Additionally, if your definition of a vacuum is the absence of all matter, then wouldn't the addition of a 10 kg mass make it not a vacuum?

Originally posted by spy66

Originally posted by SpearMint

Originally posted by spy66

Originally posted by SpearMint
Yes it would.

The answer is wrong. If i tell you that there is vacuume inside the tunnel all the way from the North pole to the South pole. How could mass travel to the South Pole? Or even to the core?

Why do you think a vacuum would stop it?

My conclusion is that the vacuume is isloleted from earth mass/gravity by the tunnel walls. If you have a equal vacuume from N to S there is no mass to cause motion to the 10kg weight. So it would never move.

You fail to grasp the most fundamental concepts of gravity. Vacuum has got nothing to do with gravity, it has no effect upon it. 10 Kg is 10 Kg no matter how much or how little air is surrounding it.

There seems to be an issue with the concept of gravity. It does happen often. When calculating, we use the "center of gravity", but that doesn't mean that all the gravitational force of an object is located at the center of it, that's merely a "trick" used in calculation. The concept of gravity that we use for the earth is the "average gravitational force exerted by the earth as a whole in relation to the surface of the earth". It's much shorter and easier to say "earth's gravity" than to spill out the entire sentence, so the concept gets misinterpreted for an experiment such as the one listed here.

All mass has a gravitational force due to its mass. We think of it as being concentrated at the center of the mass, but it isn't. If we go within the mass, each particle of mass that makes it up has a gravitational force and they all pull in different directions in relation to where the object is. The first few times you try to think it through, your brain goes a little haywire and you start wondering how it can stay in a solid mass to begin with. Gravity is one of the weakest forces we have even though, due to the mass of our planet, it's the one we feel the greatest effect from at times. We are exposed to it 100% of the time and it's foundational to the way things work for us.

Wow....I was tempted to answer some the ludicrous statements here. But after reading the many replies it becomes quite clear that the ignorance of physics is bigger than I ever thought possible.

what is very very worrying is that this ignorance is used to assess all sorts of data such as flouridation, vaccines, global warming, new energy devices etc etc Sheeeeesh!!!!!!!!!

reply to post by yorkshirelad


we need to try to correct the misconceptions as opposed to attacking them

it would eventually remain in the center point of the planet, it might have enough force to reach the other pole but it would yo-yo a bunch of times

Unless some genious can show where all the liquid hot magma has flung out of the core to leave the earth completely hollow with a thin outer crust

Once the object left the north pole, then traveled all the way to the south pole, wouldnt the same force that propelled it from N -> S then propel the object back toward N?

Also, from my understanding gravity at the center of the earth would be essentially zero, getting incrementally stronger the more the object travels away from center.

With zero friction, in a perfect vacuum, i would think this would cause the object to travel back and forth from N to S and S to N in perpetual motion like manner.

DC

Originally posted by AthlonSavage
it would stop in the middle...the field differential in centre = 0....it be suspended.
interia = 0.

I agree with this answer.

As the weight travels towards the center of the Earth, the mass of the Earth begins to accumulate all around it. The increasing mass at vectors above it's origin will serve to counter-act the gravitic force of the mass of the Earth that is still below. When the weight reaches the center of the earth, assuming it is the gravitic center, the mass will stop, because there will be an equal force of gravitic attraction from the mass of the Earth surrounding it in all directions. Theoretically, there is 'no gravity' at the center of the Earth, so it should stop and stay suspended there.

wouldn't it make sense it would go to the middle and stay there.

where the force or gravity pushing down from the north pole would collide with the force of gravity pushing down from the south pole until the cancel each other out.

the ball would be sandwiched in with no where to go.

reply to post by obviate

Theoretically, there is 'no gravity' at the center of the Earth, so it should stop and stay suspended there.

.
True, effectively no gravity at the center of the Earth but there is no reason for the ball to stop. Not without atmospheric drag.
The ball would continue to increase its velocity toward the Earth's center but the rate of change (32ft/sec2 at the start) would decrease because the mass above the ball would increase as the ball descends, counteracting the mass below. There is nothing to slow it down until it passes the center.

Once it reaches the center it's momentum would carry it the same distance it fell back toward the opposite surface. It would slow to a stop and begin falling back in the other direction. This would continue (in a vacuum) indefinitely.

It's a ballistics problem, not really very different from firing a projectile vertically, except that it will stop after a single "bounce" at its apex and returning to where it started (the ground).