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.

Abstract: A violin bow guide has a rod (38) which is attached to the back of a bow (58). The rod (38) travels through a rod guide (36) which is held in place over the strings of the violin on a support (11) which attaches to the body of the violin. The rod guide (36) is pivotally mounted to permit the bow (58) to play on all the strings. Means are provided for adjusting the height of the rod above the strings, the position of the bow between the bridge and fingerboard of the violin, and the amount of resistance to movement of the rod through the rod guide. The rod guide (36) can be locked at a selected height above the strings or permitted to float freely up and down.