"The definitions of the work of a force and a couple have been presented in terms of actual movements expressed by differential displacements having magnitudes ds and dq. Consider now an imaginary or virtual movement which indicates a displacement or rotation that is assumed and does not actually exist. These movements are first-order differential quantities and will be denoted by the symbols ds and dq (delta s and delta theta), respectively." p 551

FIRST-ORDER DIFFERENTIAL QUANTITY

JOULE
A UNIT FOR MEASURING ENERGY

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VIRTUAL WORK

WORK