Conservation of angular momentum is a physical property of a spinning system such that its spin remains constant unless it is acted upon by an external torque; put another way, the speed of rotation is constant as long as net torque is zero.
Angular momentum, also known as spin, is the velocity of rotation of something around an axis. Gyroscopes are simple devices that exploit the conservation of angular momentum to stabilize, guide or measure rotational movement in many types of systems. The law of conservation of angular momentum explains why a toy gyroscope or a spinning top remains upright as it turns rather than submitting to the force of gravity and toppling over.
Wheels on a bicycle act like gyroscopes as they spin up to speed, making it easier for the bicycle to stay upright and making it harder for anything to upset its momentum. The ability of a figure skater to increase his spin by pulling his arms closer to his body is another example of the conservation of angular momentum at work, as is the increase of an orbiting planet's spin as it gets closer to the sun.
Conservation of angular momentum is one of four exact conservation laws in physics, which state that a specified property of a given physical system remains constant even as that system evolves over time. The three other exact conservation laws are conservation of linear momentum, conservation of energy and conservation of electric charge.
Khan Academy provides a tutorial on the conservation of angular momentum: