# Why moon has no gravity ?

What are the properties constitute to have zero gravity in moon. Why earth has gravitational force while moon does not have? I need a concrete scientific explanation.

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Integral
Staff Emeritus
Gold Member
The moon does have gravity. Notice the pics of the Apollo astronauts walking about. Haven't you seen the video of the dropped hammer and feather experiment?

What are the properties constitute to have zero gravity in moon. Why earth has gravitational force while moon does not have? I need a concrete scientific explanation.
The moon does exert a force on other objects that have mass. In the parlance of our times, it does have gravity. Now the moon does not have an atmosphere... is that what you meant?

Doc Al
Mentor
http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html" [Broken]

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What are the properties constitute to have zero gravity in moon. Why earth has gravitational force while moon does not have? I need a concrete scientific explanation.
The moon's gravitational force is weaker than the earth's because gravitational force is directly proportional to mass, and the moon's mass is smaller than the earth's.

sophiecentaur
Gold Member
Don't forget that g actually depends on distance from the centre of the planet (radius) and the density.

anything with mass has gravity

Don't forget that g actually depends on distance from the centre of the planet (radius) and the density.
density?

HallsofIvy
Homework Helper
Well, it depends on the mass and that is the product of volume (which depends on radius) and the density.

density?
Yes, density determines where the surface of the mass is and the resultant gravity. If we create a new planet with higher density than the Earth with the same mass but with the size of the Moon, this object would have higher gravitational pull on its surface than both the current Moon and Earth. Because you're close to the centre.

sophiecentaur
Gold Member
This is one of the many examples in Science where the actual FORMULA says it all and much better than words.

What are the properties constitute to have zero gravity in moon. Why earth has gravitational force while moon does not have? I need a concrete scientific explanation.
You have been wildly misinformed sir. The moon has gravity, about 1/6 the gravity that earth has. Any thing with mass (everything really) has gravity. Even light has it's own gravity (since light is a form of energy and the mass off energy is equal to energy/the speed of light^2).

the moon does have gravity however its gravitational potential energy is much less than that on earth, due to the mass difference, since the moon has less mass than earth its gravitational pull is much weaker. earths GPE is roughly 9.8 Moons GPE is i think around 1.6~1.8

Doc Al
Mentor
the moon does have gravity however its gravitational potential energy is much less than that on earth, due to the mass difference, since the moon has less mass than earth its gravitational pull is much weaker. earths GPE is roughly 9.8 Moons GPE is i think around 1.6~1.8
Don't confuse gravitational potential energy with the strength of the gravitational field at the surface. The strength of the earth's gravitational field is about 9.8 N/kg (or m/s^2) at its surface, while the moon's is about 1.6 N/kg at its surface.

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sophiecentaur
Gold Member
Potential is -GM/r.
Field (g) is GM/r2.

M= 4pi r3 d/3
(where d is density)
So
g = 4pi G d r3/3r2
which means g = 4pi G d r/3
and the potential is -4pi G d r2/3

That says it all, doesn't it?