| Joule-second | |
|---|---|
| Unit system | SI |
| Unit of | action orangular momentum |
| Symbol | J⋅s |
| InSI base units: | kg·m2·s−1 |
Thejoule-second (symbolJ⋅s orJ s) is the unit ofaction and ofangular momentum in theInternational System of Units (SI) equal to the product of anSI derived unit, thejoule (J), and anSI base unit, thesecond (s).[1] The joule-second is a unit ofaction or ofangular momentum. The joule-second also appears inquantum mechanics within the definition of thePlanck constant.[2] Angular momentum is the product of an object'smoment of inertia, in units of kg⋅m2 and itsangular velocity in units of rad⋅s−1. This product of moment of inertia and angular velocity yields kg⋅m2⋅s−1 or the joule-second. The Planck constant represents the energy of a wave, in units of joule, divided by the frequency of that wave, in units of s−1. Thisquotient 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⋅m2⋅s−1.Dimensional Analysis of the joule-second yields M L2 T−1. Note the denominator of seconds (s) in the base units.
The joule-second (J⋅s) should not be confused withjoulesper second (J/s) or watts (W). In physical processes, when the unit of time appears in the denominator of a ratio, the described process occurs at arate. 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 speed is measured in the unit kilometerper hour (km/h). In physics, workper time describes a system'spower, with the unitwatt (W), which is equal to joules per second (J/s).
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