Derivation of the Navier–Stokes equations equation), three important concepts may be concisely written: conservation of mass, conservation of momentum, and conservation of energy. Validity is retained if formula_5 is a vector, in which case the vector-vector product in the second term will be a dyad. A general momentum equation is obtained when the conservation relation is applied to momentum. If the intensive property formula_5; considered is the mass flux (also "momentum density"), i.e. the product of mass density and flow velocity formula_17, by substitution in the general continuum equation: where formula_19 is a dyad, a special case of tensor product, which results in a second
Escape velocity alternative expression for the escape velocity formula_6 particularly useful at the surface on the body is: where "r" is the distance between the center of the body and the point at which escape velocity is being calculated and "g" is the gravitational acceleration at that distance (i.e., the surface gravity). For a body with a spherically-symmetric distribution of mass, the escape velocity formula_6 from the surface is proportional to the radius assuming constant density, and proportional to the square root of the average density ρ. where formula_14 The escape velocity "relative to the surface" of a rotating body depends on