Euler’s first law requires that changes in the center of mass momentum of a system of bodies can only occur when external impulses act on a system. We report the results of an experiment where after 30 test trials the mean value of known external friction impulses acting on a threebody system accounted for only ~ 8.2 per cent (standard deviation .037 and standard error .0068) of the increase in the final momentum of the system, leaving a ~ 91.8 per cent discrepancy. The three-body system consisted of two spheres, constrained to roll around quarter-circle barriers attached to a third body. Since the spheres were constrained from following their natural straight-line geodesic paths in the metric of flat spacetime, centrifugal real forces emerged radially outward from the center of mass of the spheres, pushing on the inner walls of the curved barriers. This caused the system to accelerate, inducing torque inertial forces on the spheres which increased their orbital angular speed. This increase in speed accounted for the ~ 91.8 per cent impulse discrepancy of the experiment, and thus, when added to the friction impulse, accounted for the total impulse that caused the increase in the center of mass momentum of the system per Euler’s first law.