Proper velocity Proper velocity In relativity, proper velocity, also known as celerity, is an alternative to velocity for measuring motion. Whereas velocity relative to an observer is distance per unit time where both distance and time are measured by the observer, proper velocity relative to an observer divides observer-measured distance by the time elapsed on the clocks of the traveling object. Proper velocity equals velocity at low speeds. Proper velocity at high speeds, moreover, retains many of the properties that velocity loses in relativity compared with Newtonian theory. For example, proper velocity equals momentum per unit mass at any speed, and therefore
Mass flow rate product is as follows. The only mass flowing "through" the cross-section is the amount normal to the area, i.e. parallel to the unit normal. This amount is: where "θ" is the angle between the unit normal formula_4 and the velocity of mass elements. The amount passing through the cross-section is reduced by the factor formula_5, as "θ" increases less mass passes through. All mass which passes in tangential directions to the area, that is perpendicular to the unit normal, "doesn't" actually pass "through" the area, so the mass passing through the area is zero. This occurs when "θ" = "π"/2: