What is Gravity

[imatfaal]

I believe Saturn's radius is approx 58,232 km and it's gravity 10.44 m/s2

earths radius is approx 6,371 km and its gravity 9.78 m/s2 ,do you see the problem with gravity being dependant on mass alone?

No not at all - put everything into

[latex]F= \frac{ -GMm} {r^2}[/latex]

 

and you get

http://www.wolframalpha.com/input/?i=%3D(6.67*10^-11*1*5.6846*10^26)%2F(60.286*10^6)^2

http://www.wolframalpha.com/input/?i=%3D(6.67*10^-11*1*6*10^24)%2F(6.371*10^6)^2

So if you put the respective planets masses, surface radii and a test mass of 1 kilogram in to the formula you get the two quoted surface gravities.

So no matter whatever the density, at the distances you have quoted the mass of planets as above would cause the same amount of gravitational attraction.

[studiot]

Tony, a quantity called mass appears in two unconnected areas of mechanics.

 

Gravitational mass

 

Inertial mass

 

Perhaps you would like to consider the triumph of science in showing that these are in fact equivalent.

[tonyj18]

You must also remember that the density of earth is only an approximation based on what we believe the earth is composed of ,at this point in time

[studiot]

You must also remember that the density of earth is only an approximation based on what we believe the earth is composed of ,at this point in time

 

Actually you have this backwards.

 

The density of the earth is inferred by dividing the mass, deduced from astronomical measurements of interactions with other heavenly bodies by the measured dimensions.

 

This is then used to refine theories about composition.

Edited March 7, 2014 by studiot

[pzkpfw]

You must also remember that the density of earth is only an approximation based on what we believe the earth is composed of ,at this point in time

 

No that's wrong. The effects of mass and gravity are well understood, (e.g. http://en.wikipedia.org/wiki/Cavendish_experiment ) so the mass of the Earth can be determined by the interactions of Earth/Sun and Earth/Moon.

 

Since the volume of Earth can be measured, that gives us density.

 

http://www.universetoday.com/47217/earths-mass/

 

http://www.universetoday.com/26771/density-of-the-earth/

 

 

Edit: snap!

Edited March 7, 2014 by pzkpfw

[swansont]

I believe Saturn's radius is approx 58,232 km and it's gravity 10.44 m/s2

earths radius is approx 6,371 km and its gravity 9.78 m/s2 ,do you see the problem with gravity being dependant on mass alone?

 

Dependent on mass ≠ dependent on mass alone

 

Gravity also falls off with the square of the distance, as you can see in the equation posted by imatfaal.

[michel123456]

 

tonyj18, to take a couple of steps back, to your OP:

It's already thought that every atom has a Gravitational effect. Every atom in your body is "attracting" every atom in the Earth. If you step off a ledge and fall down to Earth, it's also falling up to you. (Just, you know, it moves much less).

Your talk of "inner core" seems to imply you think Earths gravity all "comes from" the centre. That's not quite true, it's "coming from" all of Earths atoms; it's just convienient in most cases to assume it all originates from the centre of mass. Example: as you go deep under ground, you'll experience less acceleration from Gravity, as more of Earth is above you and less below. Example: the "attraction" of a nearby mountain can and has been measured.

 

After that, I don't really understand your leap to an "explanation" of gravity.

 

Derailing a bit: is the bold part above a correct statement? A body at the centre of the Earth would feel no acceleration from gravity?

[swansont]

Derailing a bit: is the bold part above a correct statement? A body at the centre of the Earth would feel no acceleration from gravity?

That is a true statement. For a spherically symmetric mass distribution, if you are at a radius R, the mass r > R makes no contribution. This is Gauss's law applied to gravitation, aka the shell theorem. Since mass varies as r³, if the density were constant the mass decreases faster than the distance contribution and g would vary linearly with r on the inside. Since in reality density is not constant, the actual variation is more complicated, but still decreases deep within the earth (as claimed) and goes to zero at r=0

 

http://en.wikipedia.org/wiki/File%3aEarthGravityPREM.jpg

[Strange]

this is more about what specifically is gravity or what causes gravity

 

We already have a very good theory for that. It is capable of making precise, quantitative predictions and has been confirmed to a high level of accuracy.

 

So:

1. What do you think is wrong with that theory? Are you aware of any evidence contradicting it, for example?

2. Why do you think your vague waffle is better? Are you able to make better, more accurate predictions, for example?

[tonyj18]

Hi Strange this forum is not compulsory ,it is only for people to discuss this topic in a grown up manner, if you aren't happy then don't join in the forum

[hypervalent_iodine]

Hi Strange this forum is not compulsory ,it is only for people to discuss this topic in a grown up manner, if you aren't happy then don't join in the forum

 

!

Moderator Note

In fact, Strange's response was somewhat of a reminder of the rules (specifically, these ones regarding evidence) that you agreed to upon signing up for this forum. Please answer his questions or you will face closure of this thread.

[tonyj18]

I have never questioned the calculations or equations we currently have to mathematically access Gravitational forces, and I not sure why you keep questioning this, my post was specifically asking, " What causes gravity" , we can all see the effect gravity has and we can all use the equations we currently have ,so I have no idea what this contradicting evidence is.

Edited March 9, 2014 by tonyj18

[pzkpfw]

I have never questioned the calculations or equations we currently have to mathematically access Gravitational forces, and I not sure why you keep questioning this, my post was specifically asking, " What causes gravity" , we can all see the effect gravity has and we can all use the equations we currently have ,so I have no idea what this contradicting evidence is.

 

... but your comments on "density" and "inner core" seem to imply you think current knowledge is wrong.

 

If what you seem to be implying is true, we'd be measuring different effects, so you need to explain how your ideas could be accurate, while we still get the results we do from our experiments.

[tonyj18]

As yet no one has been able to explain what gravity is, so any theory on what could cause gravity, can't be deemed to be wrong. My comment on density of the earth being an estimation is because it is an average of all of the material on the planet. But as yet we still do not know for sure what the deeper layers of the earth consist of.

 

The density of Earth is calculated by dividing the planet’s mass by its volume, then simplifying from The average density of Earth is 5,515 kg/m3. Since the average density of surface material is only around 3,000 kg/m3, we must conclude that denser materials exist within Earth's core. Seismic measurements show that the core is divided into two parts, a "solid" inner core with a radius of ~1,220 km[3] and a liquid outer core extending beyond it to a radius of ~3,400 km. The densities are between 9,900 and 12,200 kg/m3 in the outer core and 12,600–13,000 kg/m3 in the inner core.[4]

 

The inner core is generally believed to be composed primarily of iron and some nickel. It is not necessarily a solid, but, because it is able to deflect seismic waves, it must behave as a solid in some fashion. Experimental evidence has at times been critical of crystal models of the core. Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000K below those from shock laser (dynamic) studies. The laser studies create plasma and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is a plasma with the density of a solid.

 

This is an area of active research.

[swansont]

I have never questioned the calculations or equations we currently have to mathematically access Gravitational forces, and I not sure why you keep questioning this, my post was specifically asking, " What causes gravity" , we can all see the effect gravity has and we can all use the equations we currently have ,so I have no idea what this contradicting evidence is.

 

Several of your posts contradict this claim — that gravity depends on density and not mass or size ("It also appears as though gravity does not depend on mass", "I am suggesting if Gravity does not really depend on the mass or size of the object").

 

Your question about the nature of gravity is really a matter of metaphysics. Science focuses on how nature behaves. We know mass (and energy) cause gravity and it does this by warping spacetime.

[pzkpfw]

As yet no one has been able to explain what gravity is, so any theory on what could cause gravity, can't be deemed to be wrong. My comment on density of the earth being an estimation is because it is an average of all of the material on the planet. But as yet we still do not know for sure what the deeper layers of the earth consist of.

 

....

 

It's almost like you have not read post #1 of this thread. Which is odd, as you wrote it.

 

Yes, it's true that the specific make-up of the material in the inner layers of Earth is still a subject under investigation; but how does that relate to your topic? How does that invalidate any of the mainstream knowledge of gravity that's been presented in this thread?

 

And further, how do you think your ideas (which you seem to be avoiding directly stating) explain the "what is gravity" question any better than mainstream science?

Edited March 10, 2014 by pzkpfw

[tonyj18]

normally in most fields of science to calculate the specific density you would use:

ρ = m / V = 1 / v_g (1)

where

ρ = density (kg/m³)

m = mass (kg,)

V = volume (m³)

v_g = specific volume (m³/kg,)

and I believe that as we still do not know exactly what the inner layers are made of and therefore we don't know their m in kg's then an approximation has been assumed

What is the best estimate of the densities of the various layers of the Earth?

Estimates vary, but some approximate values should be as follows (in grams per cubic centimeter):

Continental Crust:    2.7 to 3.0Oceanic Crust:        3.0 to 3.3Mantle (silicates):   3.3 to 5.7 (increasing with depth?)Outer Core (liquid):  9.9 to 12.2Inner Core (solid):  12.6 to 13.0

[pwagen]

Parts of the above answer taken from http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry57.html

 

Feel free to include a source next time.

[studiot]

 

pzkpfw

It's almost like you have not read post #1 of this thread. Which is odd, as you wrote it.

 

+1 ...... I like it, particularly as I also replied to the OP earlier claim, but didn't receive a sensible answer. Posts 28, 29 & 30.

 

I also offered what was meant to be a helpful suggesion in post 27.

That also went unanswered.

Edited March 10, 2014 by studiot