Unit of action or angular momentum

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).

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Base units

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.

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Confusion with joules per second

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 *per* second (km/s). In physics, work *per* time describes a system’s power; defined by the unit watt (W), which is joule per second (J/s).

__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.