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The **joule-second** (symbol **J⋅s** or **J s**) is the product of an SI derived unit, the joule (J), and an SI base unit, the second (s).^{[1]} The joule-second is a unit of action or of angular momentum. The joule-second also appears in quantum mechanics within the definition of Planck's constant.^{[2]} Angular momentum is the product of an object’s moment of inertia, in units of kg⋅m^{2} and its angular velocity in units of rad⋅s^{−1}. This product of moment of inertia and angular velocity yields kg⋅m^{2}⋅s^{−1} or the joule-second. Planck's constant represents the energy of a wave, in units of joule, divided by the frequency of that wave, in units of s^{−1}. This quotient of energy and frequency also yields the joule-second (J⋅s).

In SI base units the joule-second becomes kilogram-meter squared-per second or kg⋅m^{2}⋅s^{−1}. Dimensional Analysis of the joule-second yields M L^{2} T^{−1}. Note the denominator of seconds (s) in the base units.

The joule-second *should not be confused* with the physical process of joules *per* second (J/s).

__Joules per second:__ In physical processes, when the unit of time appears in the denominator of a ratio, the described process occurs at a rate. For example, in discussions about speed, an object like a car travels a known distance of kilometers spread over a known number of seconds, and the car’s rate of speed becomes kilometers

__joules-second:__ To understand joules x second (J⋅s) we can imagine the operator of an energy storage facility quoting a price for storing energy. Storing 10,000 joules for 400 seconds would cost a certain amount. Storing double the energy for half the time would use the same resources, and cost the same.

**^**BIPM.*Le Système international d’unités / The International System of Units (‘The SI Brochure’)*. Bureau international des poids et mesures, eighth edition, 2006, updated 2014. URL http://www.bipm.org/en/si/si_brochure/, ISBN 92-822-2213-6.**^**Schlamminger, S.; Haddad, D.; Seifert, F.; Chao, L. S.; Newell, D. B.; Liu, R.; Steiner, R. L.; Pratt, J. R. (2014). "Determination of the Planck constant using a watt balance with a superconducting magnet system at the National Institute of Standards and Technology." Metrologia. 51 (2): S15. arXiv:1401.8160 . Bibcode:2014Metro..51S..15S. doi:10.1088/0026-1394/51/2/S15. ISSN 0026-1394.