Revolutions per minute velocity. The corresponding basic SI derived unit is s or Hz. When measuring angular speed, the unit radians per second is used. Formally, hertz (Hz) and radian per second (rad/s) are two different names for the same SI unit, s. However, they are used for two different but proportional ISQ quantities: frequency and angular frequency (angular speed, magnitude of angular velocity). The conversion between a frequency (measured in hertz) and an angular velocity (measured in radians per second) are: Thus a disc rotating at 60 rpm is said to be rotating at either 2 rad/s or 1 Hz, where the
Mole (unit) of "exactly" 12 g. However, on 16 November 2018, after a meeting of scientists from more than 60 countries at the General Conference on Weights and Measures in Versailles, France, organised by the International Bureau of Weights and Measures (BIPM), all SI Units will be defined as constants. This means that any SI Unit, including the mole, will not be defined using physical objects but rather they will be defined by constants that are, in their nature, exact. Such changes will officially come into effect on 20 May 2019. For example, one mole of hydrogen atoms, will be defined as