Momentum–depth relationship in a rectangular channel Momentum–depth relationship in a rectangular channel In classical physics, momentum is the product of mass and velocity and is a vector quantity, but in fluid mechanics it is treated as a longitudinal quantity (i.e. one dimension) evaluated in the direction of flow. Additionally, it is evaluated as momentum per unit time, corresponding to the product of mass flow rate and velocity, and therefore it has units of force. The momentum forces considered in open channel flow are dynamic force – dependent of depth and flow rate – and static force – dependent of depth – both affected by gravity. The
Moment (physics) of "n" corresponds to a different moment: the 1st moment corresponds to "n" = 1; the 2nd moment to "n" = 2, etc. The 0th moment ("n" = 0) is sometimes called the "monopole moment"; the 1st moment ("n" = 1) is sometimes called the "dipole moment", and the 2nd moment ("n" = 2) is sometimes called the "quadrupole moment", especially in the context of electric charge distributions. Moments of mass: Assuming a density function that is finite and localized to a particular region, outside that region a 1/"r" potential may be expressed as a series of spherical harmonics: The