Can a force generate rotation with moment arm=0 ?

AI Thread Summary
The discussion revolves around the mechanics of rotation and torque in celestial bodies, specifically questioning whether a force can generate rotation with a moment arm of zero. It is clarified that for two planets, such as the Earth and Moon, the net force typically acts through their centers, and a torque is necessary to change angular momentum. A key point made is that uneven mass distribution can lead to a net force not acting through the center, resulting in torque. The concept of tidal friction is also mentioned as an interesting related topic. Overall, the conversation emphasizes the importance of torque and moment arms in understanding celestial mechanics.
jonjacson
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I wait for your thoughts on this topic... and maybe then I will give you a small surprise.

edit:

Ok, there was a misconception on my side that I want to clarify changing the original title by this one:

If there are two planets, rotating one around the other, and only gravity is acting. Is it possible that between them the net force is not acting between their centers?
 
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Okay, I'll bite with the expected answer: no, since you need a torque to change the angular momentum and you need a moment arm to have a torque.
 
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pixel said:
Okay, I'll bite with the expected answer: no, since you need a torque to change the angular momentum and you need a moment arm to have a torque.

Ok, and what if I say to you that the two systems are Earth-Moon?
 
jonjacson said:
Ok, and what if I say to you that the two systems are Earth-Moon?
I'd say you need to clarify what you're talking about.
 
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Drakkith said:
I'd say you need to clarify what you're talking about.

Done!
 
Im not sure i understand what you mean, but if Earth and moon had arm=0 i don't think we would be here writing on the forum right now
 
jonjacson said:
If there are two planets, rotating one around the other, and only gravity is acting. Is it possible that between them the net force is not acting between their centers?

As far as I know, a planet or moon (or other body) whose mass is not distributed evenly will not have the net force acting through the center of its volume. This can (and does) result in a torque.
 
Drakkith said:
As far as I know, a planet or moon (or other body) whose mass is not distributed evenly will not have the net force acting through the center of its volume. This can (and does) result in a torque.

Well I was reading about tidal friction, a topic I found really interesting.

https://en.wikipedia.org/wiki/Treatise_on_Natural_Philosophy#cite_note-4

It is in volume 1, article 276, a nice read folks!
 
jonjacson said:
If there are two planets, rotating one around the other,

Small point but.. One planet doesn't orbit around the other. They both orbit around the combined centre of mass.

http://astro.unl.edu/naap/esp/centerofmass.html
 
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