What is angular motion?
Angular motion is the movement of a body in a circular path around an axis of rotation.
What is eccentric force?
Eccentric force is a force applied outside the centre of mass, resulting in angular motion.
What is a torque?
A torque is the measure of turning force applied to a body.
How does angular motion occur?
Angular motion occurs from an eccentric force being applied.
What is the longitudinal axis?
Longitudinal axis runs form the top to bottom of the body e.g. a trampolinist performing a full twist.
What is the transverse axis?
Transverse axis runs from side to side of the body e.g. a front somersault.
What is the frontal axis?
Frontal axis runs from front to back of the body e.g. a gymnast performs a cartwheel.
What is moment of inertia, how is it calculated and measured?
- Moment of inertia is the resistance of a body to rotation.
- Moment of inertia= mass x distribution of mass from axis of rotation²
- Measured in kilogram metres² (kgm²).
What is angular velocity, how is it calculated and measured?
- Angular velocity is the rate of rotation.
- Angular velocity= angular displacement/time taken.
- Measured in radians/second.
What is angular momentum, how is it calculated and measured?
- Angular momentum is the quantity of angular motion possessed by a body.
- Angular momentum= moment of inertia x angular velocity.
- Measured in kilogram metres² per second
What are the factors affecting the size of moment of inertia of a rotating body?
- Mass
2. Distribution of mass.
How does mass affect the size of moment inertia of a rotating body?
- The greater the mass, the greater the moment of inertia, the lower the mass, the easier it is to change the rate of rotation
- Sports with a high degree of rotation (high board diving) are typically performed by athletes with low mass
How does distribution of mass affect the size of moment of inertia of a rotating body?
- The further the mass moves from the axis of rotation, the greater the moment of inertia
- Movements where mass is tucked in around the axis of rotation have lower moment of inertia e.g. a tucked somersault
- When performing a tucked front somersault, the body will face less resistance to rotation and therefore will rotate more quickly compared with a straight front somersault.
How does moment of inertia affect angular velocity?
- If moment of inertia is high, resistance to rotation is also high, therefore angular velocity is low (slow rate of spin)
- If moment of inertia is low, resistance to rotation is also low, so angular velocity is high (fast rate of spin)
How does angular momentum apply to an ice skater performing a twist?
- At take off, angular momentum is generated by the ice skater applying an eccentric force from the ice to the body
- Rotation starts about the longitudinal axis
- Distribution of mass is away from longitudinal axis, so moment of inertia is high and angular velocity is low. Skater goes into the jump rotating slowly with control
- During flight, mass is distributed close to the longitude axis, so moment of inertia is decreased and angular velocity increases. Skater spins quickly, allowing several rotations in the air
- In preparation for landing, mass is distributed away from the longitudinal axis, so moment of inertia increases and angular velocity decreases. Skater decreases their rate of spin, increasing control for landing
- As they are landing, the ice applies an external torque to remove the conserved quantity of angular motion.
How does angular momentum apply to a diver performing a backward rotation into the water?
- At take off, angular momentum is generated by eccentric force from the springboard acting on the body
- Rotation starts about the transverse axis
- Straight body position distributes of mass is away from transverse axis, so moment of inertia is high and angular velocity is low. Diver rotates slowly with control
- During flight, the tucked body position distributes mass close to the transverse axis, so moment of inertia is decreased and angular velocity is increased. Diver rotates quickly
- Preparing to enter the water, the straightened body distributes mass away from the transverse axis, so moment of inertia increases and angular velocity decreases. Rate of spin decreases, gaining control on entry
- Angular motion is conserved throughout the movement.
