Originally Posted by otoman
BTW, acceleration seems to have been misused a little in this thread. Not only is it a change in the rate, but it is also a change in the direction of movement. Therefore, the rim is always accelerating because of the change in direction of movement of any given location on the rim.
Yes, velocity is a vector quality, and therefore so is acceleration. Both tems imply a direction and a magnitude. This is for linear systems. The concept of "direction" as a vector component takes on a differnet form when it comes to rotational systems.
Acceleration when applied to rotational systems is called angular acceleration and is the rate of change of angular velocity. Angular velocity is expressed in terms of angle/unit time (usually radians/sec). Therefore a (theoretical) wheel moving at a constant speed is *not* accelerating, at least according to Newton's Second Law of for Rotation.
But enough of this, why arent we all out on our bikes!