# Where is a Hollow Object's Center of Gravity? [closed]

Let's imagine that the Earth's moon is hollow and the remaining mass is structurally strong enough to retain its spherical shape (like a really big tennis ball).

Given this scenario, where would its center of gravity be? What would the conditions be like inside? Also would one still be able to stand on the moon?

• Amusingly, the astrophysicist Shklovsky suggested in the 1950s that Mars moon Phobos might be a hollow sphere. Indeed, today it is estimated that about 30% of its volume is made up of empty cracks/gaps. en.wikipedia.org/wiki/… May 22 '14 at 8:05
• Craig Constantine's answer below is correct. With your new edit, just disregard the part about collapse. The zero-G environment is a consequence of Newton's shell theorem.
– pela
Sep 20 '17 at 20:49

If by "a hollow object" you mean a spherical shell -- a theoretically perfect sphere Moon, with a perfect sphere of material removed from the center: The center of mass, and therefore the center of gravity remains at the geometric center. Ref Newton's Law; Bodies with spatial extent.

The body of the moon is plastic; Meaning the gravitational forces are strong enough to reshape the material... it basically flows into a sphere. So if you magically removed a small amount of the center, it would collapse into a smaller sphere. If you removed most of the center, then the "thin" shell may or may not be strong enough to support it's shape.

Inside a hollow spherical shell, there is no net gravitational force. So you would be in a zero-G environment everywhere within the shell. From outside the shell, the gravity is the same as if all the remaining mass was located at a point at the geometric center.

• +1, but your answer could be improved with an explanation of, or a reference to, Newton's shell theorem, which explains the zero-G environment. Also, note that the OP updated his question so that your second paragraph on the collapse is no longer necessary.
– pela
Sep 20 '17 at 20:51

I'm not an expert but I'll share a few ideas about it:

1. Since the space inside of the moon would be empty there would not be any kind of pressure so the moon would end up collapsing to it's center. For example thermal pressure due to particle interactions inside a molecular cloud prevents it from collapsing.
2. In the case the moon doesn't collapse, at it's center there would be no gravity at all since all the forces applied from every single point of the sphere would cancel.
3. It's a very weird situation. I think it would be possible but it would depend on how thick the sphere surface is. It the gravity at a point + the gravity at the far opposite is enough to pull you i guess yes, it's possible.

I hope this gives some clues and helps.

EDIT: from all the above statements I think the most plausible one is first one.

• Thanks, what about the edit? Any ideas? May 21 '14 at 13:46
• You're welcome, I edited the answer with it. May 21 '14 at 13:53