# Gravitational force

by kalidas1992
Tags: force, gravitational
 P: 11 I had this doubt right from my school days. What is actually gravity? what causes it? is that the core of a planet or any other forces? What creates the gravitational forces.?
 Mentor P: 15,167 Mass, any mass, not just the core of a planet, causes gravity.
P: 11
 Quote by D H Mass, any mass, not just the core of a planet, causes gravity.
If this was the answer then why dont we produce gravitational force? Human body is a mass too right sir..?

P: 844
Gravitational force

 Quote by kalidas1992 If this was the answer then why dont we produce gravitational force? Human body is a mass too right sir..?
Human body does produce gravitational force.
PF Gold
P: 272
 Quote by kalidas1992 If this was the answer then why dont we produce gravitational force? Human body is a mass too right sir..?
We do have our own 'gravity'. You exert a pull on the Earth and the Earth pulls you. The Earth has a lot more mass than you do, so it's pull is much stronger than yours.
 P: 387 There was a famous experiment where the gravitational force of a mountain in Scotland was measured: http://en.wikipedia.org/wiki/Schiehallion_experiment You'd need something at least as big as a mountain given that the Earth's gravity dominates our enviornment and makes most other gravitational forces difficult/impossible to detect.
P: 1,481
 Quote by TumblingDice We do have our own 'gravity'. You exert a pull on the Earth and the Earth pulls you. The Earth has a lot more mass than you do, so it's pull is much stronger than yours.
I just checked with may bathroom scale.
I weigh in at 180 pounds.
Turning the bathroom scale upside down ( not necessary - for illustration purposes ), the earth weighs in also at 180 pounds.
PF Gold
P: 272
 Quote by 256bits Turning the bathroom scale upside down ( not necessary - for illustration purposes ), the earth weighs in also at 180 pounds.
You're too funny !
 Sci Advisor PF Gold P: 9,445 Gravity is the allergic reaction of space to the presence of mass/energy.
P: 162
 Quote by kalidas1992 I had this doubt right from my school days. What is actually gravity? what causes it? is that the core of a planet or any other forces? What creates the gravitational forces.?
I seem to have read that gravity isn't really a force - is the shape of space around a mass.
PF Gold
P: 272
 Quote by Pjpic I seem to have read that gravity isn't really a force
That's incorrect. Gravity is one of the four fundamental forces of physics. Better reading here:

Hyperphysics: Fundamental Forces
 P: 844 Well,Pjpic and TumblingDice,each of you is correct in a sense.Gravity is one of the fundamental interactions,in that its one of the fundamental reasons that causes us to see matter change velocity.But Gravity is not a force,in that its an emergent phenomenon emerging from the shape of Space-Time.
 Sci Advisor PF Gold P: 9,445 Defining force is an exercise in futility. It is equally valid to define gravity as a distortion of space time geometry. It works the same either way.
PF Gold
P: 272
 Quote by Shyan Well,Pjpic and TumblingDice,each of you is correct in a sense.Gravity is one of the fundamental interactions,in that its one of the fundamental reasons that causes us to see matter change velocity.But Gravity is not a force,in that its an emergent phenomenon emerging from the shape of Space-Time.
Let's not get involved with a Newtonian vs. GR debate. If you want to teach that gravity is not a force, that's your choice.

Space (Pjpic) and space-time (Shyan) are not the same. All four dimensions are required for geodesics and gravitation.
P: 153
 Quote by kalidas1992 I had this doubt right from my school days. What is actually gravity? what causes it? is that the core of a planet or any other forces? What creates the gravitational forces.?
Gravity is best described as curving of space-time for now^^... In GR-Field equation, mass, energy and momentum generates gravitational effect often described as stress-energy tensor, stress-energy-momentum tensor, etc via observation Einstein-Thirring-Lense effect or dragging effect in rotating mass like a gyroscope.

Any object exerts gravity; The denser the mass the greater the effect but somewhat insignificant in the quantum level than the other forces/phenomenons, in the order of influence/strength.

http://www.edu-observatory.org/physi...periments.html
P: 5,632
 What is actually gravity?
Like any natural phenomena, gravity is what we observe. We represent such phenomena via models so we can predict some interactions that we haven't yet observed or cannot observe.
But there are aspects of 'gravity' which are not so clear.

Here are some model illustrations and some of what we observe:

Gravity is unique in that it we find it affects everything, even space and time. Other forces, like electromagnetism for example, only affect charged particles [objects]. But gravity affects charged particles.

In special relativity,SR, no gravity, relative motion causes 'space contraction' and 'time dilation', distortions in space and time.

In general relativty,GR, we find also that differences in gravitational potential also affect the passage of time.

 The "amount of gravity produced by an object" is frame-invariant; it doesn't matter what your state of motion is relative to the object.
 the boundary between gravity and geometry is indistinguishable
It was explained to me some years ago in these forums you can picture world lines [paths, or curves] in special relativty as you would curves on a flat graph paper. When gravitational curvature is involved, as in GR, the graph paper itself on which the curved worldlines are drawn is itself curved.

PAllen:
 One concept of gravity is called 'tidal gravity': even if a moon is in free fall or orbiting a massive body, different parts of it are pulled in different directions so it is under stress: tidal stress. This is the sense of gravity that, in GR, corresponds to curvature and is not a coordinate dependent feature...
On the other hand 'gravity' has some rather indeterminate features in our models: In general we can't define the energy of the gravitational field nor do we have a single overall measure
for what we call 'gravitational curvature'. Worse is that our model equations seem to break down when space time curvature becomes extreme, that is very high energy situations, like at the moment of the Big Bang and the center of black holes. If we knew everything we'd like to know about gravity, we'd have a theory of quantum gravity...meaning an overall theory and math that includes general relativity and quantum mechanics. That's a work in progress.
P: 28
 Quote by 256bits I just checked with may bathroom scale. I weigh in at 180 pounds. Turning the bathroom scale upside down ( not necessary - for illustration purposes ), the earth weighs in also at 180 pounds.
Wouldn't it be accurate to say that if you weigh 180 pounds on Earth, the Earth weighs 180 pounds on you?
P: 5,632
 Wouldn't it be accurate to say that if you weigh 180 pounds on Earth, the Earth weighs 180 pounds on you?
not really, depends on just what you mean.

'weights' vary from place to place, from reference to reference, masses don't.

Better to say they experience the same gravitational attraction, F = GMm/r2

When you start talking about WEIGHT, W = mg, and so while mass m is constant everywhere, the gravitational strength g differs...so your weight differs, say between earth and moon. g is smaller on the moon, so you 'weigh' less.

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