*a-sub-t* = *r* times* alpha*

A point on a rotating object goes around in a circle. (The
point "goes" while the object spins.) The radius *r*
of that circle is the distance from the object's axis of rotation
to the point. If the spin rate of the object is changing (meaning
the object has some angular acceleration alpha) then the speed of
the point is changing. The rate at which the point is speeding up
along its circular path (a negative quantity if the point is
actually slowing down) is called the tangential acceleration *a*-sub-t
of the point. This equation states that the tangential
acceleration of the point is equal to the radius of the circle
times the angular acceleration of the spinning object.

**Note:** A point or an object that is going
around in a circle always has centripetal acceleration. If the
point or object is also speeding up or slowing down then it also
has tangential acceleration.