# Do some scientists create equation that arent based on si units.

jaydnul
The base units are meters kg and seconds, right? So are some equations meant to use a different set, like cm kg minutes or something. And is it always assumed to be si unless told otherwise?

salzrah
any equation's variables can be converted from one unit to another; no equation is "meant to use a different set" but can be written in another way. In most physics textbooks only SI units are used.

voko
In certain cases equations might be simpler in some particular system of units. A particularly famous one is that where the units are such that all the fundamental constants (Planck's constant, speed of light, gravitational constant) equal 1 and simply omitted from the equations. These units, however, are completely unsuitable for practical use. A more practical example is the Gaussian units, which partially overlap with SI. In these units, equations of electrodynamics are somewhat simpler, so many texts use them. In non-relativistic classical mechanics equations are usually independent of the choice of units.

In any case, anyone using any equation ought to mention assumptions on the units.

Homework Helper
Gold Member
People use non-SI units every day. Classic example would the fuel consumption of your car being specified in miles per gallon.

However you should never assume any particular units have been used. Always check and recheck..

http://en.wikipedia.org/wiki/Mars_Climate_Orbiter

However, on September 23, 1999, communication with the spacecraft was lost as the spacecraft went into orbital insertion, due to ground based computer software which produced output in Imperial units of pound-seconds (lbf-s) instead of the specified metric units of Newton-seconds (N-s). The spacecraft encountered Mars at an improperly low altitude, causing it to incorrectly enter the upper atmosphere and disintegrate.