12.1.1—The radian and express angular displacement in radians

One radian is the angle at the centre subtended by an arc whose length equals the circle’s radius; a full turn is 2π radians.

Use θ=s/r for angular displacement, with arc length s and radius r in consistent units.

An arc of 0.50 m on a circle of radius 2.0 m subtends θ=0.25 rad.

Radians are not centimetres, and 360° must be converted to 2π rad before using rotational equations.