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 to escape the gravitational influence, and that there will be no future sources of additional velocity (e.g., thrust), which would reduce the required instantaneous velocity. For a spherically symmetric, massive body such as a star, or planet, the escape velocity for that body, at a given distance, is calculated by the formula where "G" is the universal gravitational constant (), "M" the mass of the body to be escaped from, and "r" the distance from the center of mass of the body to the object. The relationship is independent of the mass of the object escaping the massive body. Conversely,