This document discusses the concept of resultant forces in engineering mechanics. It defines resultant force as the single force that can replace a system of multiple concurrent forces and produce the same effect. The document explains that the resultant force is calculated using the parallelogram law, which states that drawing the diagonal of a parallelogram with sides representing the individual forces gives the magnitude and direction of the resultant. An example using two forces P and Q illustrates constructing the parallelogram and determining the magnitude and direction of the resultant force R. The magnitude of R is calculated as the square root of P^2 + Q^2 + 2PQcosα, where α is the angle between forces P and Q.