1. PHED 3 Exercise Physiology Angular Momentum
A2 PE
PHED 3
Exercise Physiology
Angular Momentum

2. Angular Momentum
Angular momentum is defined as: angular velocity x moment of inertia
The angular momentum of a system remains constant throughout a movement provided nothing outside the system acts with a turning moment on it. This is known as the Law Conservation of Angular Momentum. In simple terms this means that if a skater, when already spinning, changes their moment of inertia (by moving their arms out or into their side) then the rate of spin will change but the angular momentum will stay the same.
Figure skaters spend a lot of time either spinning on the ice or rotating through the air. The faster a skater can spin, the more impressive that spin will be to the judges. Additionally, in the skater's short program, there is a minimal required number of revolutions a skater must complete in his or her spins. To spin fast, or for a long time, the skater must develop a large amount of angular momentum.

3. Conservation of Angular Momentum
Once a skater has generated angular momentum, external forces may act to reduce the angular momentum of the skater. However, in situations, where there are no external forces producing torque about the axis of rotation, the skater's angular momentum will remain constant. This is called the conservation of angular momentum and holds true for any object.
While we illustrated this concept with a figure skating spin, angular momentum is not actually conserved in a spin due to the friction of the ice. In other words, even though a skater can increase and decrease his or her angular velocity by changing body position, as the spin progresses the skater's angular momentum gets smaller. Eventually, no matter the skater's body position, he or she will come to a stop.
However, during jumps, when the skater is rotating in the air, his or her angular momentum is conserved. This means that however much angular momentum the skater generated during take-off (by applying forces to the ice), he or she can not change it in the air. This is a very critical concept for skaters to understand, because they need to generate enough angular momentum so that they will be able to complete their jumps.

4. AQA Question
Using: - Angular momentum is defined as: angular velocity x moment of inertia
A disc of moment of inertia 10 kg m2 rotates at an angular velocity of 20 rad/s.  What is its angular momentum?
Angular momentum = 10 kg m2 x 20 rad/s = 200 kg m2 s-1.