Abstract: A device for producing a net force against a base comprising a mass change object, an accelerator, a power source operatively connected to the mass change object and configured to selectively apply to the mass change object (1) a mass-increasing waveform, characterized in that the time rate of change of the power of the mass-increasing waveform is positive, and (2) a mass-decreasing waveform, characterized in that the time rate of change of the power of the mass-decreasing waveform is negative; the power source being configured to apply the mass-increasing waveform to the mass change object when the acceleration of the mass change object has at least a component opposite to the net force direction, and to apply the mass-decreasing waveform to the mass change object when the acceleration of mass change object has at least a component in the net force direction; wherein the mass-increasing waveform is a different waveform, as a function of time, than the mass decreasing waveform.