Walter & Group.......
In regards to angular acceleration:
"# With rotatory motion there necessarily must always be acceleration, since
a rotating object is constantly changing direction.
One example is an airplane propeller. It may be rotating at a constant number
of revolutions per second, yet it is always accelerating."
A more
precise statement would be "It may be rotating at a constant number of
revolutions per second, yet it any given point
on the propeller
is always accelerating since it is constantly changing
direction."
This comes down
to our chosen frame of reference for the system we are observing. If I look at
the propeller strictly from a
linear motion
point of view then any point (with the exception of points on the axis of
rotation) must be accelerating because
they are
constantly changing direction. If we look at the propeller system from the point
of view of angular motion then once
the propeller
reaches a point of constant angular velocity the angular acceleration =
0.
This is one
of the "simple" answers. I've been trying to avoid more complex concepts such as
the fact that angular velocity
isn't a
true vector but rather a
pseudovector.
I’m really not sure what you are saying here("must always be
acceleration"). Unless you are talking about something like Coriolis
acceleration or centripetal acceleration, angular velocity can be constant
(angular acceleration = 0).
Centripetal
acceleration is correct in this case and is one of the other "simple"
answers. A discussion of centripetal and centrifugal forces
would be an
excellent addendum to the casting
physics articles...
Walter
Eric