4X01:
Kinesiology

Learning Outcomes
• By the end of this presentation you should be able to:
– Explain the concept of moment of inertia
– Outline the influence of moment of inertia on sport and exercise movements

Moment of Inertia
• Moment of inertia is the reluctance of an object to change its rotational state of motion.
• Moment of Inertia is dependant on the mass of an object and how that mass is distributed relative to the axis of rotation
I=
S m i r i
2

Moment of Inertia

Radius of Gyration
To help visualise the concept of moment of inertia sometimes the idea of a ‘radius of gyration’ is used. The is the idea that the mass of an object can be visualised as being located in an imaginary ring about the axis of rotation

Radius of Gyration

Demonstration using turntable

Principles
• Distributing mass away from the axis of rotation increases the moment of inertia.
• Distributing the mass close to the axis of rotation reduces the moment of inertia.

Examples in Sport
• Ice skating
• Somersaults in gymnastics
• Twisting in diving
• Questions
1. A sprinter is often told to flex their knee more during the swing phase in order to enable them to bring their leg through faster. Why?
2. Why does a gymnasts ability often decrease as they grow larger?