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The momentum density of a thin disc galaxy is composed of a constant velocity profile and a mass density that drops off exponentially with distance from the center. A relativistic force density obeys the product rule:

A derivative with respect to spacetime may in the classical limit become a derivative with respect to space or time. This classical gravitational law:

appears consistent with outer velocity and mass density data for the galaxy NGC3198. The product rule may eliminate the need for dark matter to explain motion of disc galaxies.