Hacker News Reader

If a pen is dropped on _a_ moon, will it

Hey, the pen really could float away, no? The centrifugal force on a rotatating body is mv^2/2. If this is exactly equal to the gravitational force, the pen would "stay where it is". And if it rotates faster than that, the pen would "float away".

View on HN

The centrifugal (pseudo-)force is not mv^2/2 (kinetic energy), but mv^2/r (much less). And the centrifugal force must be orders of magnitude less than the gravitational pull: if not, the moon wouldn't be there in solid state, to start with, as all its particles would separate from its center of mass. Not very plausible.

Reply on news.ycombinator.com