# Can a force generate rotation with moment arm=0 ?

• B
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.

jonjacson
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?

Drakkith
Staff Emeritus
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.

jonjacson
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 dont think we would be here writing on the forum right now

Drakkith
Staff Emeritus
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.

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!

CWatters