IB Syllabus focus: 'Angular motion is produced by eccentric force acting at a distance from the centre of mass. Angular momentum is conserved when free of additional eccentric forces.'
This subtopic explains how rotation begins in sport and why spinning performers continue to rotate in the air unless an external turning force changes their motion.
Producing angular motion
In sport, angular motion occurs when a body or body segment rotates around an axis. Rotation does not begin randomly; it starts because a force creates a turning effect on the body. This happens when the force is applied away from the center of mass rather than directly through it.
A force (torque) diagram emphasizing that rotation depends on the point of application of forces, not just their size. Forces whose lines of action do not pass through the pivot/axis create a moment arm and therefore a turning effect (torque), whereas forces acting through the pivot produce no turning effect. Source
Practice Questions
FAQ
This happens because the body can redistribute angular momentum internally between segments.
For example:
the arms may rotate one way
the legs or trunk may rotate the other way
The total angular momentum of the whole body stays the same if no external eccentric force acts. This is why athletes can adjust body orientation in the air without creating new total rotation.
Angular momentum depends on which axis the body is rotating around.
A performer may rotate around:
a transverse axis in a somersault
a longitudinal axis in a twist
a frontal axis in a cartwheel-type action
The same body position may feel very different depending on the axis because the mass is distributed differently around each one. That changes how easily rotation occurs and how quickly the performer can spin.
Yes, but usually only a little in many sporting situations.
Air resistance is an external force, and if it creates a turning effect, it can slightly change angular momentum. In short flight phases, this effect is often small enough to ignore. In longer aerial movements, or when body position creates more drag on one side, the effect may become more noticeable.
So conservation is often treated as an excellent practical rule, but not always a perfect one.
Beginners often apply force in the wrong direction or through the wrong part of the body.
Common issues include:
poor timing of extension
force passing too close to the center of mass
unstable posture at takeoff
limited coordination between arm swing and leg drive
As a result, they may create forward movement but not enough turning effect. Skilled performers are better at directing force so that rotation starts efficiently.
Whole-body angular momentum refers to the total rotational motion of the entire body system.
Segmental angular momentum refers to the rotational motion of individual parts, such as the arms, legs, or trunk.
These can change relative to each other during movement. For example, an athlete may swing the arms quickly while another segment responds in the opposite direction. The segments change, but the total for the body remains constant unless an external eccentric force acts.
This distinction helps explain complex aerial control in sports like diving and gymnastics.
