角速度と変位

Angular displacement is the change in angular position of a particle moving in a circular path, measured as Δθ in radians. One radian equals the angle where the arc length matches the radius, equivalent to 57.3 degrees. Angular displacement quantifies how far an object has rotated around a fixed center point.

Angular velocity measures the rate of change of angular displacement in radians per second, represented by omega. All points on a rotating object share the same angular velocity. However, linear velocity differs because points closer to the rotation center travel shorter distances than points farther away, resulting in different linear speeds despite identical angular velocities.

Angular velocity is measured in radians per second (rad/s), also called the rotation rate. Counterclockwise rotations are defined as positive, while clockwise rotations are negative. When rotation rate is given in revolutions per second, multiply by 2π to convert to radians per second, since one complete revolution equals 2π radians.

All points on a rotating rigid body sweep out the same angle in the same time interval, making their angular velocities identical. This is a fundamental property of rigid body rotation: the angular position changes uniformly across the entire object regardless of distance from the rotation axis.

Angular position, denoted θ, is the angle swept by a position vector from the circle's origin to the particle, measured in radians. The angle increases counterclockwise as the particle moves along its circular path. Angular position serves as the reference frame for measuring angular displacement and angular velocity.

A radian is defined as the angle subtended when the arc length equals the radius. This dimensionless relationship means radian measure is a ratio of length measurements. Since 2π radians equal 360 degrees, the radian provides a natural unit connecting angular displacement to the circular path's geometry.

Instantaneous angular velocity is the limit of average angular velocity as the time interval approaches zero (Δt → 0). This calculus-based definition provides the angular velocity at a specific moment, enabling analysis of rotation with changing angular velocity, which extends beyond uniform circular motion.