# The Naked Scientists Forum

### Author Topic: Is gravity and inertia the same  (Read 17614 times)

#### Jon Francis

• Jr. Member
• Posts: 28
##### Is gravity and inertia the same
« on: 14/08/2010 07:49:17 »
If space is expanding everywhere then the space between and within two objects would be expanding. Could gravity be resistance to that movement? If the two objects maintain their distance apart the units of measure between them relative to the units of measure away from them would shrink. A person living on one of the objects would see space expanding as they look away from the other object whilst the other object would seem fixed in position. A person watching from another position in the expanding space away from the two objects would think that the two objects were moving closer together. Because space has three dimensions a linear expansion in one direction becomes an accelerating expansion when viewed in three dimensions. Eventually space would be expanding faster than the speed of light relative to the observer left behind leaving a sphere of space past which they could no longer see. For the person in the expanding space they would eventually reach a point where looking back the two objects would disappear when the light could no longer reach them.

#### defkhan1

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• Posts: 6
##### Is gravity and inertia the same
« Reply #1 on: 23/08/2010 16:28:32 »
I too think gravity and inertia are related...more in the manner  of electromagnetics though.  Gravity is an acceleration of sorts, whereas inertia has to do with a steady state of motion.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Is gravity and inertia the same
« Reply #2 on: 23/08/2010 22:19:55 »
Gravity and inertia are not the same.  An object with mass will possess the same amount of inertia regardless of whether or not it's in a gravitational field, thus gravity is not a factor in inertia.

You can even see how gravity doesn't affect inertia here on Earth by simply throwing a dart at a dartboard.  The force of gravity acts downwards, through the floor, towards the center of the Earth but when you throw the dart at the dartboard the dart is travelling at right angles to the force of gravity (although, of course, it's trajectory will be bent towards the Earth), but it still has enough inertia to stick in the dartboard.  The same would happen if an astronaut, in orbit on the ISS, threw a dart at a dartboard: the dart would be effectively weightless but once again, it would still have enough inertia to embed itself in the board.

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Is gravity and inertia the same
« Reply #3 on: 23/08/2010 23:56:42 »
Gravity and inertia are not the same.
To a certain extent and for a precise reason, I disagree. I'm in good company too. :)

The relationship between gravity and inertia was the motivation for general relativity is expressed in an article Einstein wrote which appeared in the February 17, 1921 issue of Nature [28]
Quote
Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.

#### LeeE

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• Posts: 3382
##### Is gravity and inertia the same
« Reply #4 on: 24/08/2010 19:56:44 »
Methinks old Albe was having a bit of an off day when he wrote that - centrifugal force indeed!

#### Vern

• Neilep Level Member
• Posts: 2072
##### Is gravity and inertia the same
« Reply #5 on: 24/08/2010 21:03:11 »
MeThinks as well.

#### Jon Francis

• Jr. Member
• Posts: 28
##### Is gravity and inertia the same
« Reply #6 on: 29/08/2010 01:21:12 »
Correct me if I am wrong but is it not momentum the dart possesses? rather than inertia. I thought inertia was the force required to move a body whilst momentum is the energy that a moving body possesses? What I was imagining was a body held at rest by the force of gravity. Mass is all around, near and far. When a body is at rest the gravitational forces must be in balance? Like a ball held by elastic cords, where each cord is stretched by a different amount but where all the cords tension balances out. When a force acts on the body it has first to overcome all the gravity that opposes the direction of movement before the body can move.

#### Geezer

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##### Is gravity and inertia the same
« Reply #7 on: 29/08/2010 07:11:13 »
Correct me if I am wrong but is it not momentum the dart possesses? rather than inertia. I thought inertia was the force required to move a body whilst momentum is the energy that a moving body possesses? What I was imagining was a body held at rest by the force of gravity. Mass is all around, near and far. When a body is at rest the gravitational forces must be in balance? Like a ball held by elastic cords, where each cord is stretched by a different amount but where all the cords tension balances out. When a force acts on the body it has first to overcome all the gravity that opposes the direction of movement before the body can move.

I've never been able to distinguish momentum and inertia. Probably just laziness on my part.

#### Bored chemist

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##### Is gravity and inertia the same
« Reply #8 on: 29/08/2010 10:35:01 »
Gravity has a direction, conventionally called "down".
Inertia does not.
Therefore they are distinct.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Is gravity and inertia the same
« Reply #9 on: 29/08/2010 14:34:20 »
Correct me if I am wrong but is it not momentum the dart possesses? rather than inertia.

Momentum is a vector quantity, because it has a direction, due to the fact that it's moving, as well as a magnitude, whereas inertia lacks a direction and is a scalar quantity, just having a magnitude.  Momentum then, is a relative quality (because motion is relative) but we would consider any mass to have inertia regardless of its motion, so inertia is not a relative term.  The two are closely linked though.

The dart will have momentum then, relative to us, by virtue of its motion (in the direction of its motion), but it is the inertia of the dart that resists any change to that motion (in any direction), so while the dart is in flight it will have both inertia and momentum, but the momentum is only relative to us; if we were to through two darts, side by side, from the darts' point of view, neither would have momentum, relative to each other, but both would still stick in the dartboard.

When the dart is thrown horizontally there is nothing, apart from the resistance due to the air it flies through, to slow it down, so the dart's inertia continues to carry it forwards.  However, the Earth's gravity is pulling the dart down towards the Earth, at right angles to its flight path, and the dart's inertia acts here too, which is why the dart doesn't fall straight to the ground: the dart's inertia is resisting any change of motion in every direction.

When the dart hits the dartboard the material of the dartboard will try to stop the dart, changing its motion, but the inertia of the dart will try to resist that change of motion, causing it to embed itself in the board.

#### SuperPrincipia

• Jr. Member
• Posts: 31
##### Is gravity and inertia the same
« Reply #10 on: 31/08/2010 01:30:41 »
Gravity is definitely different from Inertia.

Gravity is simply defined as the attraction of mass towards mass. This mechanism by which mass is drawn towards other mass is called gravity. Actually Gravity attracts all mass towards their common centers of mass.

Inertia is simply defined as the resistance that a mass body has to resist changes in its state of motion. Either being at rest or in uniform motion.

This would mean that there are two forms of inertia
1st form of inertia defined by the mass (m):
2nd form of inertia defined by the square of the velocity (v˛)

www.SuperPrincipia.com [nofollow]

#### yor_on

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##### Is gravity and inertia the same
« Reply #11 on: 07/09/2010 20:13:51 »
The momentum will be defined from the force with which you threw those darts, as well as its mass. Potential energy and momentum differs in that potential energy always will be a definition of a combination of effects, like you smashing up your car against an obstruction. If you have a choice between crashing into the car meeting yours, or smash that enormous stationary pile of pillows resting on the side-row, you will find quite distinctly different potential energies developing, but the momentum you and your car had would be the same in both cases.

And inertia? If you are in deep space following a trajectory, no heavenly bodies in the closest light-years, you still will be the instant victim to inertia. Change the rockets course 90 degrees and you will find what I mean. So inertia is bound to you, and your combined mass, not to anything outside it. It's rather weird, inertia, I think :)
==

One way to make sense of inertia would be that theory in where gravity is like a syrup we trudge through, consisting of some sort of 'invincible' particles fastening upon us to a greater or smaller extent, depending on our motion and mass. As that then could mean that those would exist everywhere even though not getting 'activated' except when we introducing a motion.. Don't remember the name of it, the theory I mean, but Terry Pratt would like it :) And maybe it's correct too?
« Last Edit: 07/09/2010 20:26:45 by yor_on »

#### Farsight

• Sr. Member
• Posts: 396
##### Is gravity and inertia the same
« Reply #12 on: 08/09/2010 00:27:08 »
Gravity and inertia are rather different, but momentum and inertia aren't nearly so different. The distinction between them is all down to who's moving. This is the old cannonball in space thing. Is it hard to decelerate the cannonball because it's moving fast and it has considerable momentum, or is it hard to accelerate it because you're moving fast and it has considerable inertia? Motion is relative, and that's what relativity is all about.

You might not see this, and say instead that the cannonball has mass. But replace that cannonball with a spherical mirror full of photons. Those photons travel at c, and they're never at rest so we say they have no rest mass. And since rest mass is what people say mass is, we say they have no mass. But they do have momentum, and when they're bouncing around inside the spherical mirror they are at rest "in aggregate" with respect to you. Hence they add inertia to the system, and because they're there, that spherical mirror is harder to move. It really is.

See Light is heavy, which is all about a photon in a mirror-box. The photon has momentum, and it's moving at c. But when it it's going round and round it isn't moving in aggregate with respect to you, and that momentum now looks like inertia. Hence it's harder to move the system it's in. An electron isn't that different to this, it's like a photon in a box, but without any box. A cannoball is much the same, multiplied by a zillion.

How can you tell that this is how it is? Annihilate the electron with a positron, and what you've got is two photons. Annihilate the cannonball with an antimatter cannonball, and what you've got is two zillion photons. They're "out of the box", and once they've gone, there's nothing left.

« Last Edit: 08/09/2010 00:29:41 by Farsight »

#### Ron Hughes

• Sr. Member
• Posts: 363
##### Is gravity and inertia the same
« Reply #13 on: 08/09/2010 01:03:05 »
I disagree and defy you to prove any difference in gravity and inertia. The cause for the force of inertia and the force of gravity are identical. Time dilation due to mass for gravity and the force applied to a mass causing it to accelerate. If the two have the same acceleration then the time dilation is identical.

#### Farsight

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• Posts: 396
##### Is gravity and inertia the same
« Reply #14 on: 08/09/2010 14:05:14 »
The cause of the force of gravity and inertia are different, Ron. Gravitational mass and inertial mass are the same, but the cause of the force of gravity is a concentration of energy tied up as the matter of a planet. This "conditions" the surrounding space, creating a gravitational field. In that gravitational field, a cannonball falls down. If however that cannonball is way out in space where there's no discernible gravity, when you try to push it, it resists your efforts. Because the cause of inertia is the energy tied up in the cannonball itself.

You can say that the force exerted by a falling cannonball is the force of gravity. But all you've really got is a cannonball that's moving, exerting momentum that's hard to resist. In general relativity this cannonball isn't actually accelerating, instead the principle of equivalence says that it's you accelerating when you're standing on the surface of the earth. Equate this to special relativity when you're up in space, and it's like you're accelerating towards a motionless cannonball. When you catch it, you don't feel its momentum, you feel its inertia. You feel the same resistance either way because it's the same cannonball whichever way you look at it. That's why gravitational mass and inertial mass are the same.
« Last Edit: 08/09/2010 14:07:03 by Farsight »

#### Ron Hughes

• Sr. Member
• Posts: 363
##### Is gravity and inertia the same
« Reply #15 on: 08/09/2010 22:14:57 »
Your right, but if I'm in a spaceship that's accelerating at one G then your cannon ball will fall exactly the same way. Why, because the time dilation is the same.

#### The Naked Scientists Forum

##### Is gravity and inertia the same
« Reply #15 on: 08/09/2010 22:14:57 »