whose pivot is situated at the coordinate ðx0; y0Þ, assuming a rectangular coordinate system. The coordinates of the bob
in a fixed frame of reference are given by ðx; yÞ¼ðx0 þ lsin h; y0 þ l cos hÞ. The equation of momenta of the forces with
respect to the position of the bob is given by
Fxl cos h Fy lsin h ¼ 0; ð1Þ
where Fx and Fy are the components of the force acting on the pivot. The simple application of Newtons law gives
m€x ¼ Fx;
m€y ¼ Fy þ mg; ð2Þ
and introducing the coordinates of the bob with respect to the origin into the last equation, we obtain
€x ¼ €x0 l _
h2 sin h þ l€h cos h;
€y ¼ €y0 l _
h2 cos h l€h sin h: ð3Þ
Now, substituting Eq. (3) into Eq. (2), and then into Eq. (1), we get the following general equation of motion for the
simple pendulum
l€h þ g sin h þ €x0 cos h €y0 sin h ¼ 0; ð4Þ
where it is assumed that the functions €x0ðtÞ and €y0ðtÞ are known (as a matter of fact they represent the equations of
motion of the supporting point) and the coordinate h is defined, as usual, in the interval p 6 h 6 p. This equation can
be also obtained using Lagrangian methods by simply considering the Lagrange function
L ¼ 1
2
mðx_
2 þ y_
2
Þ þ mgy ¼ 1
2
mðx_
2
0 þ y_
2
0 Þ þ ml _
hðx_ 0 cos h y_0 sin hÞ þ 1
2
ml2 _
h2 þ mgy0 þ mgl cos h: ð5Þ