Understanding the Discrepancy: Why Does the Moon Have Less Gravity Than Earth?

  • Context: Undergrad 
  • Thread starter Thread starter Pierre Ordinaire
  • Start date Start date
  • Tags Tags
    Earth Gravity Moon
Click For Summary

Discussion Overview

The discussion revolves around the differences in gravitational acceleration between the Earth and the Moon, specifically addressing why the Moon, despite having 1/81 the mass of the Earth, exhibits only 1/6 of Earth's gravity. The scope includes conceptual understanding and technical reasoning related to gravitational forces and densities of celestial bodies.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question the relationship between mass and gravity, noting that the Moon's lower mass results in weaker gravity, but they seek clarification on the expected gravity value.
  • One participant explains that gravity is influenced by both mass and distance from the center of the body, suggesting that if one were at a significant height on the Moon, gravity would be weaker than on Earth.
  • Another participant references the density of celestial bodies, indicating that if the Moon had the same density as Earth, its gravity would be closer to 1/4 of Earth's rather than 1/6.
  • Clarifications are made regarding the interpretation of mass comparisons, specifically that the reference is to the Moon's current mass, not Earth's.
  • A mathematical expression for gravitational acceleration is presented, prompting further calculations.

Areas of Agreement / Disagreement

Participants express differing views on the implications of mass and density on gravitational strength, and there is no consensus on the expected gravity value or the interpretation of density effects.

Contextual Notes

Some assumptions regarding density and mass relationships are not fully explored, and the mathematical implications of gravitational calculations remain unresolved.

Pierre Ordinaire
Messages
3
Reaction score
1
TL;DR
Gravity on Earth vs the Moon.
Apologies in advance if this has been touched before. If the Earth has 81 times the mass of the Moon, why the Moon has 1/6 of the gravity of the Earth?
 
Astronomy news on Phys.org
Do you find that strange? What do you think it ought to be instead of 1/6?
 
  • Like
Likes   Reactions: Vanadium 50
Acceleration due to gravity increases with mass and decreases with distance squared from the center (outside spherically symmetric bodies, anyway). The moon is less massive but much smaller. If you built a tower on the moon so high that you were as far from the center as the radius of the Earth, gravity there would be 81 times weaker than the surface gravity if the Earth. But "on" the moon usually means on the surface, much closer to the center than on a huge tower.
 
  • Like
Likes   Reactions: vanhees71 and etotheipi
Thank you.
 
  • Like
Likes   Reactions: Ibix
Pierre Ordinaire said:
Summary:: Gravity on Earth vs the Moon.

Apologies in advance if this has been touched before. If the Earth has 81 times the mass of the Moon, why the Moon has 1/6 of the gravity of the Earth?
See:
https://phys.org/news/2016-01-strong-gravity-planets.html

For example: Saturn has 95 times more mass than Earth, but about the same surface gravity:
 
  • Like
  • Wow
Likes   Reactions: etotheipi and vanhees71
A.T. said:
See:
https://phys.org/news/2016-01-strong-gravity-planets.html

For example: Saturn has 95 times more mass than Earth, but about the same surface gravity:
Of course that is due to the fact that Saturn has a density that is 1/8 that of the Earth's.
In the same vein, The Moon has a lower density than the Earth. If it had the same density as the Earth and the same mass, its surface gravity would be closer to 1/4 the Earth's rather than 1/6.
 
  • Like
Likes   Reactions: PeroK
Janus said:
If it had the same density as the Earth and the same mass
I assume that means "the same mass as the actual Moon". Not "the same mass as the Earth" - then it would be identical. Obvious if you think about it, but at a first reading it looked wrong.
 
mjc123 said:
I assume that means "the same mass as the actual Moon". Not "the same mass as the Earth" - then it would be identical. Obvious if you think about it, but at a first reading it looked wrong.
Yes, I meant the moon's current mass.
 
g=GM/R^2. Work that out.
 

Similar threads

Undergrad How our Moon and Sun may affect Earth's equatorial bulge
  • · Replies 3 ·
Replies
3
Views
1K
High School The Sun's Influence on the Moon's Orbit: Is it Negligible?
  • · Replies 4 ·
Replies
4
Views
3K
Undergrad Nature paper on Moon's initial evolution - Tidally driven remelting
  • · Replies 1 ·
Replies
1
Views
3K
High School Earth & Moon Gravity Gradient: Investigating Its Impact
  • · Replies 33 ·
2
Replies
33
Views
5K
Undergrad Does gravity affect aging on different planets?
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad How Does the Sun's Position Affect the Earth-Moon Barycenter?
  • · Replies 4 ·
Replies
4
Views
6K
Undergrad Why are Saturn's moons so bright?
  • · Replies 2 ·
Replies
2
Views
2K
High School For Lack of a Better Term....Moon Motion
  • · Replies 18 ·
Replies
18
Views
3K
Undergrad Does the barycenter of the Earth-Moon system change?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Why Is Jupiter's Surface Gravity Not as High as Expected?
  • · Replies 10 ·
Replies
10
Views
3K