MKS Units

It is very helpful, when working with several equations, to have them all expressed in exactly the same units.

It is not enough to say “all the choices must be metric”. Specific units must be chosen. For example, we might say that all values of length should be in meters. An equation using centimeters would require us to do a multiplication or division by 100 to change it over to meters.

The MKS system uses: meters, kilograms, seconds

These choices make it so the following are all true:

  • units for Force will be Newtons N = kg \dfrac{m} {s^2}
  • units for Energy will be Joules J = N m = kg \dfrac{m^2} {s^2}
  • units for Power will be Watts W = \dfrac {J} {s} = \dfrac {N m} {s} = \dfrac {kg \: m^2} {s^3}

Units are very important, just ask the people who were responsible for the $125,000,000 Mars Climate Orbiter that missed its target because numbers calculated using English were given to people who expected numbers calculated using metric.

Can you see that if we agree to use metric but one of us is using centimeters and the other is using meters, in some way we’ll be off by a factor of 100?