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A gravitational field is generated by a mass off the bottom of the image. Tidal forces are the forces experienced by pieces of the depicted body with respect to whole of that body (in other words, tidal force is the difference between the force on a particular point within the body and the average force experienced by the entire body). Thus, tidal forces are calculated by subtracting the value of this field at the center of a "moon" - the center of the circle illustrated. This leaves as a residual the forces illustrated.
Note that the forces have been calculated from the more accurate (1/r2 - 1/r02) version, not the l/r3 approximation. The tidal forces are not quite perfectly symmetrical about the center of the moon.
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