23
$\begingroup$

Momentum and energy play very similar roles in mechanics, each being changed by the application of force over a interval. For energy the interval is in space and for momentum it is in time. Both have associated conservation laws.

Yet, energy units are named in many systems and momentum units generally are not (and in particular are not named in SI).

Is there a clear historical reason for the difference?

Improve this question
$\endgroup$
4
  • 6
    $\begingroup$ I don't know where you go the impression that there is no SI unit for momentum. It's called the Mom, 1 Mom=1 kg.m/s. The milliMom is also known informally as the "Mommy," and the kiloMom as the "big Mama." $\endgroup$
    – user466
    Commented Oct 22, 2015 at 14:35
  • 9
    $\begingroup$ I can't tell if you're joking... :) $\endgroup$
    – Ne Mo
    Commented Oct 22, 2015 at 16:33
  • 4
    $\begingroup$ @Ne Mo: Joking? Why would I be joking? Also, the megaMom is nicknamed the "mother of all momenta." Use of the microMom is discouraged, because it's so small -- it shouldn't be Mom until it grows up. $\endgroup$
    – user466
    Commented Oct 23, 2015 at 15:55
  • $\begingroup$ There is also no special name for the unit for the torque, or "moment of force". The SI unit is the Newton metre, which of course is quite different from the Joule, since here the metre is orthogonal to the Newton, and to have a Joule you need a metre parallel to the Newton... $\endgroup$
    – Alfred
    Commented Jan 10, 2022 at 6:31

2 Answers 2

Reset to default
21
$\begingroup$

There is a historical reason. But it was not a fluke of history, the underlying reason is that energy comes up in non-mechanical (thermal, electric) contexts whereas momentum does not. Derived alternative, newton-meter in SI, did not arise naturally in such contexts, and alternative units, like calories, were used prior to the discovery of the general energy conservation law. It made sense to replace them with named standardized units. Klein's Science of Measurement is a nice historical survey.

Standardization and naming of units happened in two waves. First started during the French revolution at the behest of Taleyrand (known as bishop turned politician who later managed to serve as foreign minister under both Napoleon and Bourbons after Waterloo). French Academy appointed a committee on weights and measures, including Condorcet, Lagrange, Laplace, Monge, and unofficially Lavoisier before he was guillotined, came up with the foundation of SI. At that point only the base units of length, time, weight and temperature received single names. The derived units of area, volume, speed, acceleration, work/energy, torque, momentum, etc., were combinations of the base ones even if they had commonly named units like liters or knots. Future kilogram, called kilograve, was the unit of weight, i.e. force, not mass. Mechanical energy and momentum received equal treatment.

The second wave, started in 1860s and formalized by 1880s in both SI and its competitor CGS, was meant to catch up with developments in thermodynamics and electromagnetism, and gave us ohms, volts, farads, watts, etc. Kilograve renamed into kilogram became the unit of mass, the unit of force was named dyne in CGS (from Greek dynamis - force), and newton in SI. What of energy? Its mechanical manifestation received no new attention, but in 1864 Clausius suggested erg (from Greek ergon - work) to replace calorie as a unit of thermal energy, a remnant of by then discredited caloric/phlogiston theory. It was adopted in CGS. The unit for power, watt, was suggested even before joule, by Siemens in 1882, to replace Watt's own horsepower used to measure output of steam engines. Siemens was an electric engineer. Joule himself was honored by a unit name for determining the mechanical equivalent of heat. Momentum was out of luck.

Improve this answer
$\endgroup$
2
  • $\begingroup$ This is a mix of fact and error. Some clear-cut errors: 1. The initial name of dm^3 of water was grave, not kilograve (see e.g. here). But grave was dispensed with by 1795 in favor of the gram and the kilogram. 2. The 1795 law introduced also the are (100 m^2), stère (1 m^3), and litre (1 dm^3). 3. It is absurd to say that in the 1860s-1880s the SI and CGS were 'competitors'. There was no SI then, only 'practical electric units'. $\endgroup$
    – linguisticturn
    Commented Jul 29 at 3:52
  • $\begingroup$ Pace Klein, it is not clear that Condorcet et al. sought to define a unit of force. Here is a 1771 (shortened) viewpoint on this: The mass of a body is known by weight; if we have two bodies, the first of which weighs two or three times as much as the second, the first has two or three times as much mass as the second. By mass measurements, I mean pounds, ounces, etc., that mass weighs. In measurements of weight or mass, we can take the ounce as a fundamental unit. $\endgroup$
    – linguisticturn
    Commented Jul 29 at 4:38
9
$\begingroup$

In 1887 a committee of the British Association was appointed for the purpose of "considering the desirability of introducing uniform nomenclature for the fundamental units of mechanics of co-operating with other bodies engaged in similar work.” The committee issued a series of questions to members, and collected their replies. The result was that in 1888, when the committee met at Bath, they were able, amid much difference of opinion, to agree as to the desirability of introducing names for the C. G. S. units of velocity, momentum, and pressure; the names suggested being, kine, bole, and barad respectively.

Improve this answer
$\endgroup$
3
  • $\begingroup$ This is very interesting as some of my education was conducted in cgs units but even there no one mentioned a named unit for momentum. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Feb 16, 2017 at 22:53
  • 1
    $\begingroup$ Somehow the proposed cgs units kine and bole were not very succesful. In 1891 the electrical engineer and inventor William Henry Preece said "They tried to thrust upon us the term barad as indicative of the standard of pressure, bole the unit of momentum, and kine the unit of velocity, but I think this attempt has practically failed, and I have met but one case where one of these names was used." By the way, bole might be derived from a Greek word meaning throw, as in hyperbole and parabole. $\endgroup$
    – jkien
    Commented Feb 19, 2017 at 22:27
  • $\begingroup$ SI does have a unit for pressure, namely the Pascal (Pa). But when I think, it is always in Newtons per squared meter. Then at the end, I just use the conversion table: 1 N/m^2= 1 Pa... $\endgroup$
    – Alfred
    Commented Jan 6, 2022 at 21:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.