For a rigid slab in translation, show that the system of the effective forces consists of vectors \left( \Delta { m }_{ i } \right) \overline { a } attached to the various particles of the slab, where \overline { a } is the acceleration of the mass center G of the slab. Further show, by computing their sum and the sum of their moments about G, that the effective forces reduce to a single vector m\overline { a } attached at G.