The power number N_p (also known as Newton number) is a dimensionless number relating the resistance force to the inertia force. In engineering, this number, along with the Reynolds number, is one of the most widely employed dimensionless numbers. In dimensional analysis, a dimensionless number (or more precisely, a number with the dimensions of 1) is a pure number without any physical units; it does not change if one alters ones system of units of measurement, for example from English units to metric units. ... In physics, resistance force is the force which an effort force must overcome in order to do work on an object. ... Inertia is the tendency of any state of affairs to persist in the absence of external influences. ... ... The Reynolds number is the most important dimensionless number in fluid dynamics and provides a criterion for determining dynamic similitude. ...

The power-number has different specifications according to the field of application. E.g., for stirrers the power number is defined as:

with

• P: power
• ρ: fluid density
• n: rotational speed
• d: diameter of stirrer