8bf0eb34_PMM_physics__About_friction__Type
[...and something about it] An effect which is frequently underestimated. Followers of perpetual motion often refer to the celestial sphere as an example of mechanical perpetual motion. A motion which is not so perpetual as we think. Have you ever wondered why the moon always shows the same face? The reason is tidal friction. The tidal forces caused and cause these effects: The moon was braked down due to the inner friction of the semi-liquid core, until the tidal force was no longer effective. This is exactly the case when the moon's rotational period is identical to the time it needs to revolute once around the earth. The tidal forces of the moon partly heat the inner earth because of inner friction of the earth's core. Because of this, and the friction by the tidal water-hills, the rotational speed of the earth slows down. [Type] 
tidal friction
s l i d i n g f r i c t i o n
r o l l i n g f r i c t i o n
e l a s t i c d e f o r m a t i o n
p l a s t i c / i n e l a s t i c d e f o r m a t i o n
l i q u i d f r i c t i o n
e l e c t r o m a g n e t i c f r i c t i o n
t i d a l f r i c t i o n
s o u n d
PMM physics. About friction
Type