The radian per second (symbol: rad·s−1 or rad/s) is the SI unit of rotational speed (angular velocity), commonly denoted by the Greek letter ω (omega). The radian per second is also the unit of angular frequency. The radian per second is defined as the change in the orientation of an object, in radians, every second.

Angular frequency ω (Ordinary) frequency
2π radians per second exactly 1 hertz (Hz)
1 radian per second approximately 0.159155 Hz
1 radian per second approximately 57.29578 degrees per second
1 radian per second approximately 9.5493 revolutions per minute (rpm)
0.1047 radians per second approximately 1 rpm

Since the radian is a dimensionless unit, the radian per second is dimensionally equivalent to the hertz—both are defined as s−1. This may lead to confusion between the quantities angular frequency ω and frequency ν.

A use of the unit radian per second is in calculation of the power transmitted by a shaft. In the International System of Units, widely used in physics and engineering, the power, p, is given by the product of ω (in radians per second) times the torque, τ, in newton-meters applied to the shaft. Thus, p = ωτ, and the unit is the watt, with no numerical coefficient needed. In other systems, an additional factor may be necessary. For example, if one multiplies angular velocity in revolutions per minute (rpm) times the torque in pound-feet, then a multiplicative constant is needed to give the result in horsepower.

An angular frequency, ω = 1 rad/s, corresponds to an ordinary frequency, ν = 1/(2π) Hz ≈ 0.159 Hz, which in turn corresponds to a frequency of rotation of 60/(2π) rpm ≈ 9.55 rpm.

See also