質量行列
2 T = m x ̇ 2 + m y ̇ 2 + m R 2 α ̇ 2 − 2 R d sin  ( α ) x ̇ α ̇ + 2 R d cos  ( α ) y ̇ α ̇ {\displaystyle 2T=m{\dot {x}}^{2}+m{\dot {y}}^{2}+mR^{2}{\dot {\alpha }}^{2}-2Rd\sin(\alpha ){\dot {x}}{\dot {\alpha }}+2Rd\cos(\alpha ){\dot {y}}{\dot {\alpha }}}
m = m 1 + m 2 {\displaystyle m=m_{1}+m_{2}} と d = m 1 − m 2 {\displaystyle d=m_{1}-m_{2}} 。この式は、次のように行列形式で記述でき
T = 1 2 q ̇ T M q ̇ {\displaystyle T={\frac {1}{2}}{\dot {q}}^{\textsf {T}}M{\dot {q}}}
M = [ m 0 − R d sin  α 0 m R d cos  α − R d sin  α R d cos  α R 2 m ] {\displaystyle M={\begin{bmatrix}m&0&-Rd\sin \alpha \\0&m&Rd\cos \alpha \\-Rd\sin \alpha &Rd\cos \alpha &R^{2}m\end{bmatrix}}}