A gravity motor utilizes a heat expansion mechanism comprising a frame, a rotational shaft supported by the frame, a drive gear connected to the rotational shaft, the drive gear having a plurality of teeth around the outer periphery thereof, a plurality of drive bars, the drive bars at one end thereof being pivotally secured to the drive gear around the circumference of the drive gear to permit pivoting of the drive bars within a predetermined range limited by two teeth of the drive gear and the other end of each of the drive bars extending radially outwardly therefrom, and a plurality of expansion means, at least one expansion means being respectively situated between the drive bars to change the distance between the drive bars, whereby when one or more expansion means is actuated continuously on one side of the motor by heating or cooling the expansion means, the motor becomes successively unbalanced between the two sides to thereby move continuously. The machine has been designed to use solar energy to drive a wheel with a minimum loss of energy. It may also be activated by electric or nuclear energy in the simplest and most practical form. It is a slow motion stationary machine and therefore long lasting requiring very little maintenance.