# Why Gravity attracts all objects with the same speed?

Why Gravity attracts all objects with the same speed? Is this question was solved? What is the exact answer?

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Could you elaborate what you mean by the question? Could you formulate it in another way? For example one could read it as "Why does gravity attract all of the following: Objects with the same speed" or one could interpret the question as "Why does gravity attract all objects, and why does it do it with the same speed?". In any case, both options don't really make sense in my ears. Do you maybe mean "Why is gravity such that all objects (of different masses) fall with the same speed if they start out not moving with respect to each other? Why does it accelerate all object equally?" – NikolajK Jul 20 '12 at 13:30
Yeah , if you dropping two cannon balls of different sizes and mass they come to ground at the same time – Emma Jul 20 '12 at 13:33
I'm sure there are previous questions that articulated this question better, but in short, relativity introduced some unifying principles that doesn't answer the question, but redirects it to something more fundamental to the nature of the universe. Firstly, it's not that gravity affects matter, it affects matter-energy, and secondly, a gravitational field is a complimentary with acceleration itself. So, matter is energy, energy is matter, gravity is acceleration, and acceleration is gravitation. Hope that clears it up for you, although I'm sure it doesn't. – Alan Rominger Jul 20 '12 at 13:48
I think its an unsolved question of physics . i search about it and i just find different answers , no of answer like you (AlanSE) Thanks – Emma Jul 20 '12 at 14:04
Possibly related: physics.stackexchange.com/q/11321/2451 – Qmechanic Jul 20 '12 at 14:04

Gravity does not attract all objects with the same speed but rather with the same acceleration. This means that any two objects in the same gravitational field will change their speeds by the same amount in any given time period. Of course, if both objects start with the same speed then their speeds will be the same as they accelerate - if you drop two objects at the same time they will land (if you can ignore air resistance, of course) at the same time.

This fact is known as the equivalence principle and has been tested experimentally to within something like one part in $10^{10}$ (though I can't find the reference and this is rather a wild guess based on the accuracy of measurements of $G$). In physics, however, you can't ever experimentally prove two quantities are exactly equal - all you can ever say is that their difference is smaller than whatever uncertainty your apparatus has.

If you take the equivalence principle at its word and take it to be exact as a postulate, then what it says is that gravity is a fictitious force like the Coriolis and centrifugal force, which are proportional to objects' mass and due to a "poor" choice of reference frame. Explorations of this idea led Einstein to formulate the General Theory of Relativity, whose predictions have been tested time and again in a number of different settings.

The main question of "why?", however, remains unanswered and probably will stay like that. Physics can only say "how" the world is but every answer to "why" will always be something you can counter with "yes, but why?".

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+1 for mention of the fictitous force. General relativity is a science built on the idea that the correspondence between inertial and gravitational mass is more than a mere coincidence. It is a deep consequence of the fundamental construction of spacetime. What that construction is exactly, that's a little more difficult. – Alan Rominger Jul 20 '12 at 18:15
Thanks for your answer , can you give me an example of a phenomenon in physics that we can explain the "how this phenomenon occur" AND "why this phenomenon occur" ? – Emma Jul 20 '12 at 23:31
Great answer. Nitpick, though: you can sometimes prove that two quantities are exactly equal. For example, we know that electrons are identical due to their interference. I can't think of a way to do such a thing measuring gravity, but hey, never say never. – So8res Jul 21 '12 at 6:01
"why does the nucleus stay together?" -> "well, there's the strong nuclear force that works so and so, and it's caused by gluon exchange between quarks". To which you can always ask, "ok, but why do quarks exchange gluons?" Any answer to "why" is bound to be incomplete. – Emilio Pisanty Jul 22 '12 at 10:47

OK, Ms. Galileo...

So you're having a little fun at the Torre Pendente dropping billiard balls on the turistas in Pisa. But they see them coming and get out of the way, even if you drop two together. You need something that falls faster.

So you take a pair of balls before you drop them and connect them together with a short thread and a couple drops of glue. That makes a solid twice as heavy, right?

Think that will work?

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The fact that all matter responds in the same way to a gravitational field has led Einstein to postulate General relativity .One should apply relativistic ideas to gravity .Newtonian theory of the gravitational field is not relativistically invariant. First you postulate that all matter and Energy responds the same way to a gravitational field . You then try to construct a global inertial reference frame in a gravitational field .The field strength at different points is different and you realize that it is impossible to construct a global inertial reference frame but it is possible to take a limiting procedure and construct a reference frame at each point in space . This is very similar to riemannian geometry and tells that spacetime may be a curved manifold.Einstein postulated that matter leads to curvature and matter follows the geodesics of this curvature and developed a theory whose predictions have been successfully tested experimentally.

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Because the gravitational force is proportional to the product of the masses, $F\propto m$, but the acceleration of a mass subject to a force is proportional to the force and inversely proportional to the mass, $a=F/m$, which means that the constant of proportionality in the first case is precisely $a$.

As Emilio Pisanty says, this arises from the fact that the gravitational mass (the mass you multiply and divide by the distance to get the attractive force due to gravity) seems to be exactly the inertial mass (the mass you multiply by the acceleration to get the force).

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in the Geodesic formulation of GR there is NO mas.. the mass is not important so the lenght and form of the geodesic will not depend on the mass only on how the spacetime is curved.

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