A rotating bicycle wheel has angular momentum, which is a property involving the speed of rotation, the mass of the wheel, and how the mass is distributed. For example, most of a bicycle wheel’s mass is concentrated along the wheel’s rim, rather than at the center, and this causes a larger angular momentum at a given speed. Angular momentum is characterized by both size and direction.
The bicycle wheel, you, and the lazy susan form a system that obeys the principle of conservation of angular momentum. This means that any change in angular momentum within the system must be accompanied by an equal and opposite change, so the net effect is zero.
Grace is now standing on the lazy susan with the bicycle wheel spinning. One way to change the angular momentum of the bicycle wheel is to change its direction. To do this, Grace must exert a twisting force, called a torque, on the wheel. The bicycle wheel will then exert an equal and opposite torque on Grace. (That’s because for every action there is an equal and opposite reaction.) Thus, when Grace twist the bicycle wheel, the bicycle wheel will twist Grace the opposite way. since Grace is standing on a low-friction pivot, the twisting force of the bicycle wheel will cause grace to turn. The change Grace’s angular momentum compensates for the change in angular momentum of the wheel. The system as a whole ends up obeying the principle of conservation of angular momentum.
A bicycle wheel is suspended from one of end of its axis by a rope, and spun up by hand.
The gyroscope seems to defy gravity because the torque created by the spinning wheel counteracts the torque due to gravity. Gyroscopes have been used through history for varied uses such as stabilizing spacecraft or for guidance systems on ships and missiles.