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fundamental SI Supplementary SI units other SI

There are two supplementary SI units that are used to distinguish between quantities which are of a different nature. Otherwise, they would have the same SI units. For example, angular velocity is specified in units of radians per second (rads-1). Since radian is dimensionless, without the supplementary unit the angular velocity would have specified as s-1, which is the same as that of frequency. The supplementary units are shown below:

Supplementary quantity



plane angle



solid angle




  • Radian - It is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.

  • Steradian - It is the solid angle that, having its vertex in the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.

fundamental SI other SI

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