This is part of a series on common misconceptions.
Is this true or false?
An object undergoing uniform circular motion does not accelerate.
Why some people say it's true: In uniform circular motion, speed remains constant.
Why some people say it's false: In uniform circular motion, the direction of motion is ever-changing.
To cut through the confusion, let's look at the definition of acceleration: the time rate of change of velocity. Whenever velocity changes, there must be a corresponding acceleration.
The confusion comes from the difference between velocity in 1-dimension and velocity in multiple dimensions. In one dimension, velocity has a magnitude (e.g. ) and a direction (e.g. toward the northeast). However, as the direction can only be toward the left or the right, it isn't possible to smoothly vary the direction of velocity—as is the case in circular motion—we can only have discrete shifts. Such motion isn't usually encountered except during collisions, where few would doubt the existence of significant acceleration.
In dimensions, velocity is a full-fledged vector quantity and its direction can be varied naturally. One such case is uniform circular motion where the direction of velocity varies smoothly as we move about the circle. Despite the constancy of speed, the direction of motion is changing and therefore the time rate of change of velocity is nonzero—which constitutes an acceleration.
In what direction does the object accelerate? As its speed is unchanging, the acceleration must be perpendicular to the direction of motion, and thus toward the center of the circle. Its magnitude can be found in a number of ways, and is given by where is the radius of the circle.
Rebuttal: In the case of uniform circular motion, what is the angle between velocity and acceleration?
Reply: If the speed remains constant, then the component of acceleration which is parallel to the velocity is zero. This component is called as tangential acceleration. The direction changes due to the centripetal acceleration which is radially inward. Thus, the net acceleration in the case of uniform circular motion is perpendicular to the velocity.
Rebuttal: In the case of uniform circular motion, can the magnitude of acceleration be written as equal to the rate of change of speed?
Reply: No, the rate of change of speed is entirely different from the rate of change of velocity. Acceleration is defined as the rate of change of velocity.
A particle is moving on a circular track with constant non-zero speed. Which of the following options are correct?
(a) The acceleration of the particle is zero.
(b) The rate of change of speed equals the magnitude of the rate of change of velocity.
(c) Instantaneous speed equals the magnitude of instantaneous velocity.
(d) The angle between velocity and acceleration has to be .