The velocity of an object is the rate of change of the object’s displacement in a frame of reference with respect to time; it includes the direction of motion. Speed is the magnitude of velocity without information of direction. An object is said to be traveling with constant velocity if it travels with constant speed without changing direction. An object in circular motion with constant speed does not travel with constant velocity, because its direction of motion changes with time; it accelerates toward the center of the circle, although its speed is constant. Acceleration is the rate of change of velocity; it is not the rate of change of speed. For the purpose of simplifying discussion, velocity and speed are often used interchangeably; however, we should know that they are different.