Why? Because rotating bodies are not in constant,
straight-line motion, and so forces must be acting (the deformations produce
internal forces that cause the accelerations).
If a vessel, hung by a long cord, is so often
turned about that the cord is strongly twisted, then filled with water, and
held at rest together with the water; thereupon, by the sudden action of
another force, it is whirled about the contrary way, and while the cord is
untwisting itself, the vessel continues for some time in this motion; the
surface of the water will at first be plain [plana], as before the vessel
began to move; but after that, the vessel, by gradually communicating its
motion to the water, will make it begin sensibly to revolve, and recede by
little and little from the middle, and ascent to the sides of the vessel,
forming itself into a concave figure (as I have experienced), and the swifter
the motion becomes, the higher will the water rise, till at last, performing
its revolutions in the same times with the vessel, it becomes relatively
at rest in it. This ascent of the water shows [indicat] its endeavor to recede
from the axis of its motion; and the true and absolute circular motion of
the water, which is here directly contrary to the relative, becomes known
[innotescit], and may be measured [mensuratur] by this endeavor. … There
is only one real circular motion of any one revolving body, corresponding
to only one power of endeavoring to recede from its axis of motion, as its
proper and adequate effect; but relative motions, in one and the same body,
are innumerable, according to the various relations it bears to external
bodies, and like other relations, are altogether destitute of any real effect,
any otherwise than they may perhaps partake of that one only true motion.
(Newton's
Principia trans R Rynasiewicz)
