Consider a ball tied to a string rotating in a circular trajectory. The rate of change of angular displacement is called its angular velocity. Like linear velocity, angular velocity is also a vector quantity, and a rotation in a clockwise direction is considered as the negative direction. Consider a ball rotating at high speed, the rate of change of angular displacement will be high, and hence the angular velocity will be high. The angular velocity value at any time during the motion is called its instantaneous angular velocity, and it is expressed as a derivative of θ with respect to time. The rate at which the angular velocity of an object changes is called its angular acceleration, denoted by the letter α with units of radians per second square. Angular acceleration is a vector quantity and is considered positive when the angular velocity increases and vice-versa. Since two points on the string have the same angular velocity, their angular acceleration is also the same.