<Dd> F = F 1 + F 2 = 2 (B + D 2 − A) = 2 (E − A), (\ displaystyle \ mathbf (F) = \ mathbf (F) _ (1) + \ mathbf (F) _ (2) = 2 ((\ frac (\ mathbf (B) + \ mathbf (D)) (2)) - \ mathbf (A)) = 2 (\ mathbf (E) - \ mathbf (A)),) </Dd> <P> where E is the midpoint of the segment BD that joins the points B and D . </P> <P> Thus, the sum of the forces F and F is twice the segment joining A to the midpoint E of the segment joining the endpoints B and D of the two forces . The doubling of this length is easily achieved by defining a segments BC and DC parallel to AD and AB, respectively, to complete the parallelogram ABCD . The diagonal AC of this parallelogram is the sum of the two force vectors . This is known as the parallelogram rule for the addition of forces . </P> <P> When a force acts on a particle, it is applied to a single point (the particle volume is negligible): this is a point force and the particle is its application point . But an external force on an extended body (object) can be applied to a number of its constituent particles, i.e. can be "spread" over some volume or surface of the body . However, determining its rotational effect on the body requires that we specify its point of application (actually, the line of application, as explained below). The problem is usually resolved in the following ways: </P>

What is the definition of net force in physics