
#1
Dec613, 03:08 AM

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.? 



#2
Dec613, 03:35 AM

Mentor
P: 14,459

Mass, any mass, not just the core of a planet, causes gravity.




#3
Dec613, 04:01 AM

P: 11





#5
Dec613, 04:51 AM

PF Gold
P: 207





#6
Dec613, 08:41 AM

P: 334

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. 



#7
Dec713, 07:41 PM

P: 1,260

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. 



#9
Dec813, 01:03 AM

Sci Advisor
PF Gold
P: 9,182

Gravity is the allergic reaction of space to the presence of mass/energy.




#10
Dec813, 01:09 PM

P: 162





#11
Dec813, 06:48 PM

PF Gold
P: 207

Hyperphysics: Fundamental Forces 



#12
Dec813, 10:35 PM

P: 734

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 SpaceTime.




#13
Dec913, 12:07 AM

Sci Advisor
PF Gold
P: 9,182

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.




#14
Dec913, 02:51 AM

PF Gold
P: 207

Space (Pjpic) and spacetime (Shyan) are not the same. All four dimensions are required for geodesics and gravitation. 



#15
Dec913, 04:51 AM

P: 121

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.eduobservatory.org/physi...periments.html 



#16
Dec913, 08:18 AM

P: 5,634

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. PAllen: 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. 



#17
Dec1613, 08:55 PM

P: 28





#18
Dec1713, 11:13 AM

P: 5,634

'weights' vary from place to place, from reference to reference, masses don't. Better to say they experience the same gravitational attraction, F = GMm/r^{2} 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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