'Someone' seems to be having an unusual amount of trouble understanding how a force can exist between two masses that are different. But unless you have two masses (which are therefore, by definition, different masses), you don't have a force. Since we know there are many more than two different masses (bodies with mass) around, we know there are forces between them (but we don't know why or how). We do know how to explain this in terms of the difference (in mass). So, two masses, one force between them (and between every other different mass in the entire universe). What is so hard about this? Why does "someone" keep turning one force into two forces (one that each mass somehow "has" between itself and the other mass), this is clearly wrong, and a misunderstanding. It's a coupling or a connection, not couplings or connections; you only need one spring to model this, not two.
Take the Earth and the Moon for example - both have mass, meaning both have gravity. Because of this, they both pull on each other. It's not like because the Earth is bigger, the Moon's gravity is neglected. It would also help to picture gravity as a a depression in time-space. the Moon is creating one depression, and the Earth creates another. There is the same force at work here, but there are two instances of it.
Almost. There may be two depressions, but that's the same as two masses. You say: "the same force is at work here, but there are two instances of it". You appear to have also misunderstood the principle, clearly stated above: there are two bodies of mass, there is one force.
if it is the same force with different mass i assume we are measuring in newtons, therefore you have not given the velocity. As a gravitational affect how does a fast rotation affect compared to a slow rotation. Quite simply, force. A marble that weighs one gram, and a bowling ball that weights 10 kg (exaggerated). The marble moved 1000 times faster than the bowling ball which is at one meter per second, the marble would carry the same force, if they were moving at each other in a vacuum they would both stop on impact. They have balancing forces. Onto gravity, the earth and moon in fact make something called a binary planetary system, where both magnetic fields from the moon and earth are actually combined ever so slightly. The moon and earth are one. The depression of space time can also be consider the curvature of space or also Orbit.
Sure. So how does the single gravitational attractive force between the Earth and Moon, become: "two instances". Two instances of what?
The earth and moon pull on the sun as one not two. This planet is part one of two. Therefore the gravatational curve around this planet is slightly different that a single planet with no moons. If the moon affects water, than it affects things in orbit too.
Uh huh. But how does the single attractive force between the Earth and Moon, or between the Sun and Jupiter, become "two instances"? Can you answer this?
It would be easier to consider gravitational field produced by the Earth-Moon system. This field is defined everywhere, and to compute the net force felt by, say the Earth, just integrate the field multiplied by the local mass density (assuming the Earth is absolutely rigid).
Again, almost. The field is defined everywhere, i.e. at all points, that lie between the Earth and the Moon. But could we be getting somewhere?
Hang on, if you're referring to the interaction between the two bodies, you're saying it's defined "everywhere"? Or you mean gravity is everywhere, since the Earth and Moon aren't the only two?
A single force, by definition, always acts on only one object. If it did not, then Newton's laws of motion would be meaningless. But, all forces result from interactions between two or more objects, and so forces always come in pairs. Newton's third law of motion quantifies this statement, saying that for every force on one object, there must be an equal magnitude force acting on some other object, in exactly the opposite direction. In Newtonian gravity, when the Earth pulls on the Moon with a certain force, then the Moon pulls back on the Earth. And so, we have two forces: one acts on the Earth, the other acts on the Moon. They have the same magnitude, but act on two different objects and in opposite directions.
But calling the two "forces" a single force, is incorrect? Why are there two? Why does the Moon pull back on the Earth as the Earth pulls on the Moon? Newton's 3rd Law indeed states that every (re)action has an equal opposing reaction, and a force or an impulse qualifies as a reaction (or an interaction). When the Earth pulls on the Moon, you're saying there's an equal and opposite [action or] reaction, by the Moon? Would that be the Moon's inertia, possibly? Also, why does Newton's Law of Universal Gravitation, not give two results? Should there not be two separate calculations, or his formula should, after plugging in the mass of the two bodies, have a result for each body, which would be: two answers or results, but the formula is single-valued? If there's always a pair of forces, as you say? How does this get explained then?
I mean gravitational field is everywhere, even if the Earth and moon were the only objects in the whole universe.
Checkaroony. But how do we know there is this field everywhere? What observations have led to this conclusion?
Who told you that? There is absolutely nothing anywhere in the universe that we know about, that is this thing you describe, i.e. "a single object". There is no such thing. Bearing in mind that a force is not usually (in fact it's never) perceived, objects are. You need objects for forces to be "seen" acting on them. Just thought I'd try and let you know that.
In Newtonian gravity (as it is a mathematical model) we just know. But this is already a very accurate model. Basically, Tycho Brahe's observation led to Kepler's three laws, and I think Kepler's laws were one of the biggest stepstones of Newton's theory.
Vkothii: Because for every force there is an equal force on some other object that acts in the opposite direction. Yes. This applies to any two objects interacting via gravity. For example, when you jump, while you're in the air the Earth's mass exerts a force of gravity that pulls you towards the Earth. Equally, you exert a force that tends to pull the Earth upwards towards you. Inertia is not a force. Inertia is the tendency of an object to retain its current state of motion, and is related to mass. Because the two forces acting on bodies interacting gravitationally always form a Newton's third law action-reaction pair. It's tied up with the very definition of the word "force", as I said earlier. If you want to explain interactions in some different way to Isaac Newton, maybe you can do that, but you'll have to start by defining what you mean by force, and how your definition is different from Newton's definition. Sure there is. A ball is a single object. You are a single object. An electron is a single object. A single object does not need to be indivisible. It can be made up of smaller parts, yet still be a single object. Care is often required in physics when defining a "system" to be examined. A system can be a single particle or a complex object made of billions of particles. It doesn't really matter, as long as you're consistent in your analysis of your chosen system. I'm aware of the fact that forces have no existence beyond their effects on objects.
OK. Shall I start with, say Kepler's second law? Or just go straight to the description in some undergrad text of Newton's discovery of the universal interaction between two bodies which produces a motion that Kepler's laws describe? How the force is actually central; you know what "central" means? (Hint: it's why two bodies move toward each other along a line joining them, or they orbit around a common centre of mass, both bodies see exactly the same force pushing, or equivalently pulling them along this line.) Also what the force being "exactly proportional to" the mass, or the "amount" of matter in each body (that is, not the amount in one of them, or in the other, but the total amount) means? Because I'm not convinced, looking at your responses to my questions, that you do understand what it means. Perhaps you should instead explain how your definition is the same (that is, not different to Newton's, as it looks to be from here)?
All the things you describe as "single", are only so relative to all of the other balls, people, electrons, etc around them. They can only be "single objects" because there are no indivisible, single things that are objects. We can make a single electron move, or make electrons move one at a time; we can throw balls one at a time too. Since a "single" object is not indivisible, then a single object is in fact many objects, so as I say, there is no single, independent thing anywhere, object-wise. More exactly, no singular thing.
