Добавить физическому телу постоянное ускорение, игнорируя его массу.
Apply the acceleration in each FixedUpdate over a duration of time. In contrast to ForceMode.Force, Acceleration will move every rigidbody the same way regardless of differences in mass. This mode is useful if you just want to control the acceleration of an object directly. In this mode, the unit of the force parameter is applied to the rigidbody as distance/time^2.