9.1 TYPICAL OSCILLATORY SYSTEMS
corresponding to a kinetic energy, is positive definite; that is, whatever non-zero
real values the ̇qitake, the quadratic form (9.1) has a value>0.
Turning now to the potential energy, we may write its value for a configuration
qby means of a Taylor expansion about the originq= 0 ,
V(q)=V( 0 )+
∑
i
∂V( 0 )
∂qi
qi+
1
2
∑
i
∑
j
∂^2 V( 0 )
∂qi∂qj
qiqj+···.
However, we have chosenV( 0 ) = 0 and, since the origin is an equilibrium point,
there is no force there and∂V( 0 )/∂qi= 0. Consequently, to second order in the
qiwe also have a quadratic form, but in the coordinates rather than in their time
derivatives:
V=
∑
i
∑
j
bijqiqj=qTBq, (9.2)
whereBis, or can be made, symmetric. In this case, and in general, the requirement
that the potential is a minimum means that the potential matrixB, like the kinetic
energy matrixA, is real and positive definite.
9.1 Typical oscillatory systems
We now introduce particular examples, although the results of this section are
general, given the above restrictions, and the reader will find it easy to apply the
results to many other instances.
Consider first a uniform rod of massMand lengthl, attached by a light string
also of lengthlto a fixed pointPand executing small oscillations in a vertical
plane. We choose as coordinates the anglesθ 1 andθ 2 shown, with exaggerated
magnitude, in figure 9.1. In terms of these coordinates the centre of gravity of the
rod has, tofirst orderin theθi, a velocity component in thex-direction equal to
l ̇θ 1 +^12 l ̇θ 2 andinthey-direction equal to zero. Adding in the rotational kinetic
energy of the rod about its centre of gravity we obtain, to second order in the ̇θi,
T≈^12 Ml^2 ( ̇θ^21 +^14 ̇θ 22 +θ ̇ 1 ̇θ 2 )+ 241 Ml^2 ̇θ^22
=^16 Ml^2
(
3 ̇θ^21 +3 ̇θ 1 ̇θ 2 +θ ̇^22
)
= 121 Ml^2 q ̇T
(
63
32
)
q ̇, (9.3)
where ̇qT=( ̇θ 1 ̇θ 2 ).The potential energy is given by
V=Mlg
[
(1−cosθ 1 )+^12 (1−cosθ 2 )
]
(9.4)
so that
V≈^14 Mlg(2θ^21 +θ^22 )= 121 MlgqT
(
60
03
)
q, (9.5)
wheregis the acceleration due to gravity andq=(θ 1 θ 2 )T; (9.5) is valid to
second order in theθi.