# Nature of gravity

does the effect of gravitational force vary with state of matter,i.e
the force is most felt in gases?right.
anything having mass experiences gravity,so 2 gases having masses should experience gravity between themselves.

## Answers and Replies

nicksauce
Science Advisor
Homework Helper
Yes, gasses are affected by gravity, as is everything with mass.

then moon's gravitational pull has to affect gases also,only the water bodies are affected causing tides,why ?

I think you will find that the earths atmosphere does go through tidal variations as does the solid parts of the earth.

Anything that has mass is indeed affected by gravity.

F = G(m1m2)/r^2

Gas is mass. It's not very dense, but it is still mass.

Light (low density) gases tend to rise above heavy (high density) gases. CO2, with a gram molecular weitht of 44 grams per Avagadro's # of molecules, will sink in stagnant air and sometimes asphixiate people (see http://en.wikipedia.org/wiki/Lake_Nyos). Water is about 800 times more dense than air.

jtbell
Mentor
Anything that has mass is indeed affected by gravity.

If gases were not affected by gravity, the Earth would not have an atmosphere and we wouldn't be here writing about this!

If gases were not affected by gravity, the Earth would not have an atmosphere and we wouldn't be here writing about this!

That's exactly right jtbell. I should have thought of posting that fact. That is probably the best example/proof that gases are affected by gravity. The Earth's gravity is what holds our atmosphere in place. The same is true for all celestial bodies with atmospheres. It explains why the moon and mercury do not have atmospheres. They are too small for their gravity to hold one.

Excellent point made!

mass or energy is affected by gravity.

No, only matter is affected. Gravity could be thought of as a type of energy, but a force is simply a perpetual energy exerter.

russ_watters
Mentor
No, only matter is affected. Gravity could be thought of as a type of energy, but a force is simply a perpetual energy exerter.
None of that is correct.  Well, maybe the part in the middle, but it would be a real stretch - like calling a spring a type of energy.

None of that is correct.  Well, maybe the part in the middle, but it would be a real stretch - like calling a spring a type of energy.

A compressed spring has potential energy and has more mass when compressed .

Yes, gasses are affected by gravity, as is everything with mass.
What about light.
Passing starlight has been observed to be bent by the sun yet it don't have mass only energy.

yes light is affected by gravity it also creates gravity.

it is obvious that light is affected by gravity,thats how shift occurs.
now when the moon exerts its gravitational pull on earth,it is the gases that need to
be affected first,rather the liquid bodies.but tides are being created whille atmosphere
seems to be intact.

A.T.
Science Advisor
now when the moon exerts its gravitational pull on earth,it is the gases that need to be affected first, rather the liquid bodies.
What do you mean by "first"? They are bouth affected simultaneously.
but tides are being created whille atmosphere seems to be intact.
The oceans seem to be intact as well. The height of the water surface changes by a few meters only. Since the atmosphere doesn't have a distinct upper surface it would be hard to detect such an effect on it.

yes light is affected by gravity it also creates gravity.

Is this true? Light creates gravity?

Gravity (i.e., mass or weight) is not a conserved quantity. For example, when a positron annihilates at rest with an electron, the gravity (mass) associated with the 1.02 MeV in rest mass is lost. Some measurements have been made on the gravitational attraction of low Z materials (e.g., carbon, which is half neutrons) vs. high Z (e.g., lead, which is 60% neutrons), but the measurements are not conclusive on neutron vs. proton gravitational attraction.

well," first", i meant in priority,gravitational pull would be more over gases or liquids?
according to the formula,does increase in mass mean increase in force of gravity?

sorry to post again, do we know the rate at which gravity acts?

gravity travels at the speed of light

how do we know that gravity travels at the speed of light,is this speed constant throughout?

this speed should be constant , im not sure how they know this exactly .

A compressed spring has potential energy and has more mass when compressed .

Please explain to us how a spring gains mass by being compressed.

Im not sure gravity travels. It is only a force that is determined by the masses and distance apart of two distinct objects. The greater the masses or the smaller the distance, the higher the gravity. Its not all that difficult. Does not matter if it's gas, liquid, or solid. Gravity depends on mass, not density!

"A compressed spring has more energy and hence more mass when it is compressed."
Please explain to us how a spring gains mass by being compressed.
This is an interesting topic. If I have two chemical compounds, and they are exothermic when combined, where does the exothermal energy come from? It is certainly chemical, but does this initially come from mass being converted to energy? I think dynamite is 4 kilojoules (atomic energy) per gram, and a nuclear reaction (in reactor) may be 190 megajoules (nuclear energy) per gram. Is one due to mass difference and the other due to stored chemical (atomic) energy and not mass difference? If I have a hydrogen atom with an electron in the 1s state, and another hydrogen atom in the 2s state, are the two atomic masses the same? When the 2s electron drops to the 1s state (via the 2p), where does the photon energy come from; mass difference or chemical difference? If I take two identical springs, compress one (and bind it with a string) and put both in weak sulfuric acid, I can make a battery (of sorts) out of them, so mechanical energy is being converted to chemical energy, and into electrical energy plus heat. When I take a Mossbauer Effect source and put it on top of a building (= added potential energy), the 14.7 keV gamma rays have higher energy than the other Mossbauer Effect source left in the basement. How does carrying the iron-57 source up several flights of stairs add nuclear energy to the source? Is a gravitational potential being converted to nuclear energy? Certainly the compressed spring energy is stored as interatomic strain inside the spring as distortion of the domains, but could that be really a miniscule mass difference?

Energy and mass can be considered to be two different aspects of the same thing thus we can measure energy in mass units and mass in energy units the choice of units being a matter of convenience.The total mass/energy before a chemical reaction is the same as that after but the energy converted to/from heat is the difference in mass energy between the products and reactants.More specifically it is the difference in bond energies.Taking a radioactive substance to a higher place result in an increase of gravitational potential energy conventional wisdom stating that this energy is within the field.For the majority of energy changes it is impossible to measure the energy converted by using mass measurement techniques as can be appreciated by Einsteins famous equation.Thus we can easily measure the energy stored in a compressed spring, we can calculate what its increased mass would be by dividing by c^2 but we cannot measure this increased mass by any sort of weighing technique.If we choose to we can take it on trust that it is more massive.

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This thread is riddled with mistakes and poor word choices that are causing people to be confused, with time I shall try to remember to revisit and correct some of this.

For now, I recommend to the OP that a simple start is had by reading something like:

http://en.wikipedia.org/wiki/Gravity

and revisiting this thread with any specific questions or issues that need clarification. Though, I am interested why, monty37, you feel that it is obvious that light is deflected in a gravitational field?

Otherwise, this further talk about springs is also flakey but unrelated to the OP's question.

well i said it was obvious because ,anything that has mass is affected by gravity and
so is light.well,the link was very helpful ,i would also like to know how the
speed of gravity was calculated?

Im not sure gravity travels.
It travels , what would happen if our sun immediately vaporized would the earth fall out of orbit immediately , no it would remain in orbit for as long as it would take light to travel to earth , cause nothing can travel faster than light. Plus the graviton which is mass less is believed to travel at the speed of light.