Is gravity a force that 'pulls' from the centre of the earth, or an external force that pushes down on earth?

Question

Thank you Thomas, for your words of wisdom, I knew that answer when I was 11 too!
Glad you enjoy physics!

did anyone rad the answer that mentioned the trampoline....I wanted to read it again but it seems to have vanished.

Thank you Donna M - that's the one! Where did it disappear to?

My computers been down a few days - I still have to read all these answers carefully and not sure how much time is left before it automatically goes to vote...., thank you all!

My question now is if I accept that gravity is a 'pulling force' and every action has an equal and opposite reaction, then there must be a 'pushing force' too.

The idea that gravity has a 'pushing' element to it - the universe pushing back - is not mine, I have read this theory elsewhere and wondered if anyone else had ever come across it, or knows where it came from.

Answer 1

Every bit of matter has a gravimetric pull. Usually it is negligible and unnoticable, however, on scales the size of moons, planets and stars, the combined gravity of all the matter it is made up of become significant.
In the case of the earth, it would depend upon where the most dense region is. Presumably this would be the core but we know so (comparatively) little about the inner earth that we can't be sure. For instance, if the core is hollow than all gravity would come from the matter surrounding that hollow, therefore there would be no "central" point of gravity. Even if the core is not hollow, if you start shedding layers of the earth until you got to a tiny central point, most of the gravity would have dissipated because it depends upon the existence of mass. One particle or one boulder will not warp the fabric of space-time enough to be noticeable, but on the scale of the earth, the combined matter creates a "depression" in space-time that is significant enough to pull macro sized objects into it.
So, the answer is that the earth in it's entirety provides the pull of gravity that we feel. But, I just noticed I haven't technically answered your question yet. Gravity is a force that attracts (pulls) and not a force that repels (pushes). Although if anti-gravity is ever discovered it would have a repulsion effect which is opposite to normal gravity.
Cheers.

Answer 2

You are attracted to every other molecule with mass in the universe, but since the magnitude is inversely proportional to distance, only relatively close large objects have a measurable effect.

You are not attracted to the "centre of the Earth", but rather, to every particle of the Earth's mass. If you are on the surface, or above the surface then the net effect of all the attraction from all the particles is roughly towards the centre of the Earth.

As you move away from the surface of the Earth, the force is less strong (inverse square law again). However, contrary to what is said above, below the Earth's surface, some of the particles are attracting from left, right, forwards, backwards and above, as well as "down" - the net effect of which is that only those particles "below" you affect your net gravitational pull. Hence, the closer you get to the centre of the Earth, the less the Earth's net gravitational effect is, and at the "centre" the net force due to gravity is zero.

Strangely enough, every molecule of the Earth is attracted in the same way to every molecule of you, but the effect is imperceptibly small because of the size difference. Consider, however, the Earth-Moon system:

The Moon is attracted to the Earth and is falling continuously towards the Earth; the Earth is attracted to the Moon and is falling continuously towards the Moon - only the fact that they are moving pretty fast keeps them from actually colliding. But, neither is orbiting the other - both bodies are falling towards a central point, the centre of mass (or centre of gravity). Because the Earth is so much larger than the Moon, this centre of mass is located not at the centre of the Earth, but rather a few thousand kilometres below the Earth's surface, towards the centre. If the Earth and Moon were attached by a big stick from centre to centre, they would rotate about a fixed point on that stick, much much closer to the bigger mass.

Likewise, you and the Earth attract each other, and the centre of attraction is close to the centre of the Earth, but shifted slightly towards you.

Last point of interest (promise) : experiments carried out in Scotland around the conical mountain Schiehallion show that the gravitational pull of "the Earth" is shifted slightly towards the mountain, no matter which side of the mountain your are at.

So, the answer could be: depends how close you are to a mountain...

Answer 3

Gravity is not a force that is found from the cetre of the earth. It is there in any body of mass. the earth's Centre of Gravity, the place where the overall effect of the gravitational force concentrated is at the centre of the earth. That is why things fall down. Which looks like pulling down.

Answer 4

It's a little more complicated than that. Basically, just like in electro-magnetics it is the passing of photons between particles. The faster the photons are being passed, the stronger the pull, the slower they are being passed, the weaker the pull. Most scientists believe this is being done by another partical called a graviton, unfortunately, they've never been able to find one. Why might you ask, because many scientists such as myself, do not believe they exist. We think it is a side effect of other physic principles. The problem is that the smaller and smaller you go, the more 'known physics' break down and stop making sense.

Answer 5

There are two forces in nature that we experience every day: gravity and magnetism. You may have magnets on your refrigerator, and you know that a magnet will attract a refrigerator with a certain amount of force. The force depends on the strength of the magnet and the distance between the magnet and the metal. You also know that magnets have two poles -- north and south. Either pole will attract iron or steel equally well, north will attract south, and like poles will repel one another.

Gravity is the other common force. Newton was the first person to study it seriously, and he came up with the law of universal gravitation:

Each particle of matter attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
The standard formula for gravity is:

Gravitational force = (G * m1 * m2) / (d2)
where G is the gravitational constant, m1 and m2 are the masses of the two objects for which you are calculating the force, and d is the distance between the centers of gravity of the two masses.

G has the value of 6.67 x 10E-8 dyne * cm2/gm2. That means that if you put two 1-gram objects 1 centimeter apart from one another, they will attract each other with the force of 6.67 x 10E-8 dyne. A dyne is equal to about 0.001 gram weight, meaning that if you have a dyne of force available, it can lift 0.001 grams in Earth's gravitational field. So 6.67 x 10E-8 dyne is a miniscule force. When you deal with massive bodies like the Earth, however, which has a mass of 6E+24 kilograms (see this Question of the Day), it adds up to a rather powerful force. It is also interesting to think about the fact that every atom attracts every other atom in the universe in some small way!

Answer 6

Newton explained that everything with mass is attracted to everything else with mass. The force of attraction, which we call gravity, is proportional to the product of the two masses and inversely proportional to the square of the distance between them. So, the brick you are holding and the Earth attract one another, that's why the brick feels as though it wants to rush down to the ground. The distance between two masses can often be taken for practical purposes to be the distance between their centres, so the force you feel on that brick is related to the distance to the centre of the Earth, not just to the ground you are standing on. Where one mass is very small compared to the other, such as a brick and the Earth, it seems as though the force acts only on the smaller one, because it accelerates more rapidly, but where the masses are similar in size it is more obvious that each of them is attracting the other.

Answer 7

If it were an extternal, pushing force it would be a rare coincidense that it pushed us towards Earth and not towards some point in empty space.

In fact, mass attracts mass according to the formular
gravitationalforce = mass1 * mass2 * gravitationalconstant /distance^2

So only if one (or both) masses are quite large and not too fare away, the force is significant. You don't feel the gravity of the Moon because it's too far away (and lighter than Earth) and you don't feel the gravity of a car or a dog because they are too light.

Answer 8

Gravity is a pulling force that originates from any large mass (the earth is a pretty big mass). So everything from Mountains (albeit very small force) to the sun emit gravity that effects the motion of objects.

Answer 9

any mass (that is, stuff that you can describe in terms of how many kilos it is) is attracted to other mass. The Earth has a lot of mass (there is alot of it) so the attraction between it and other masses ie us is strong. Really you and the planet are attracted to each other, but the earth is so much more massive than you that it seems like you are attracted to the Earth. This attractive force exsists between all masses even between people but is so absolutely tiny that you can't feel it, actually i have heard it said that the attractive force of gravity between two people in an intimate embrace is similar to the weight of a 2mm square of paper!

Answer 10

It is a force which all mass exerts, though as it is soooooo weak, you need a large very large mass to attract another object with any appreciable force. Every time you jump up, you are defeating gravity, and birds can manage it for hours. In our solar system, Jupiter is incrdibly massive, and the gravity there is something like twelve times ours - wh would die horribly if exposed to 12G, it seems fighter pilots struggle to handle 9 for even a few seconds.