Differentiating twice with respect to time yields an approximate expression for the acceleration of the piston as follows,

                                                                                                              (3.6)

When multiplied by the mass of the piston, this gives the inertia force due to the piston.

                                                               (3.7)

The first term signifies variation at the same frequency as the speed of rotation and hence is known as the PRIMARY force term and the second term is known as the SECONDARY force term. For a typical case, these two terms are pictorially shown in Fig 3.2