Since work is equal to the change in energy, then work and energy—potential as well as kinetic, of course—are measured in one and the same units. Work is equal to the product of a force by a distance. The work done by a force of one dyne over a distance of one centimetre is called the erg: \[1\,\textrm{erg} = 1\,\textrm{dyn} \cdot 1\,\mathrm{cm}\] This is a very small work. Such a work is performed against gravity by a mosquito in order to fly from the thumb to the forefinger of someone’s hand. A larger unit of work and energy used in physics is the joule (J). It is 10 million times as great as an erg: \[1\,\mathrm{J} = 1\times 10^{7} \, \textrm{dyn}\] A unit of work which is quite often used is 1 kilogram-force-metre (\(1\,\mathrm{kgf\,m}\)). This is the work which a force of \(1\,\mathrm{kgf}\) performs in a displacement of \(1\,\mathrm{m}\). About this much work is done by a kilogram weight falling off a table to the floor.

As we know, a force of \(1\,\mathrm{kgf} = 981000 \, \textrm{dyn}\), \(1\,\mathrm{m} = 100\,\mathrm{cm}\). Hence, \(1\,\mathrm{kgf\cdot m}\) = \(9.81\times 10^{7}\)  erg = \(9.81\,\mathrm{J}\). Conversely, \(1\,\mathrm{J} = 0.102\,\mathrm{kgf\cdot m}\).

The SI system of units requires that we drop the kilogram-force-metre as the unit of work and energy and use the joule instead, \(1\,\mathrm{J}\) is the work done by a force of \(1\,\mathrm{N}\) over a distance of \(1\,\mathrm{m}\). Knowing how easily force is defined in this case, one has no difficulty in understanding the reason for the advantages of the SI system of units.