Lagrange's Equation

The Cartesian equations of motion of our dynamical system take the form

$$m_j \ddot{x}_j = f_j,$$ (B.9)

for $j=1,{\cal F}$ , where $m_1, m_2, m_3$ are each equal to the mass of the first particle, $m_4, m_5, m_6$ are each equal to the mass of the second particle, et cetera. Furthermore, the kinetic energy of the system can be written

$$K = \frac{1}{2}\sum_{j=1,{\cal F}} m_j \dot{x}_j^{ 2}.$$ (B.10)

Now, because $x_j=x_j(q_1,q_2,\cdots, q_{\cal F},t)$ , we can write

$$\dot{x}_j= \sum_{i=1,{\cal F}} \frac{\partial x_j}{\partial q_i} \dot{q}_i + \frac{\partial x_j}{\partial t},$$ (B.11)

for $j=1,{\cal F}$ . Hence, it follows that $\dot{x}_j = \dot{x}_j(\dot{q}_1,\dot{q}_2,\cdots, \dot{q}_{\cal F},q_1,q_2,\cdots,q_{\cal F},t)$ . According to the previous equation,

$$\frac{\partial \dot{x}_j}{\partial\dot{q}_i} = \frac{\partial x_j}{\partial q_i},$$ (B.12)

where we are treating the $\dot{q}_i$ and the $q_i$ as independent variables.

Multiplying Equation (B.12) by $\dot{x}_j$ , and then differentiating with respect to time, we obtain

$$\frac{d}{dt}\!\left(\dot{x}_j \frac{\partial \dot{x}_j}{\partial... ..._i} + \dot{x}_j \frac{d}{dt}\!\left( \frac{\partial x_j}{\partial q_i}\right).$$ (B.13)

Now,

$$\frac{d}{dt}\!\left(\frac{\partial x_j}{\partial q_i}\right) = \s... ...\partial q_k} \dot{q}_k + \frac{\partial^{ 2} x_j}{\partial q_i \partial t}.$$ (B.14)

Furthermore,

$$\frac{1}{2} \frac{\partial\dot{x}_j^{ 2}}{\partial \dot{q}_i} = \dot{x}_j \frac{\partial \dot{x}_j}{\partial \dot{q}_i},$$ (B.15)

and

$$\frac{1}{2} \frac{\partial \dot{x}_j^{ 2}}{\partial q_i} = \dot{x}_j \frac{\partial \dot{x}_j}{\partial q_i} = \dot{x}_j \frac{\partial}{\partial q_i}\!\left(\sum_{k=1,{\cal... ...\partial x_j}{\partial q_k} \dot{q}_k + \frac{\partial x_j}{\partial t}\right)$$

$$= \dot{x}_j\left(\sum_{k=1,{\cal F}}\frac{\partial^{ 2} x_j}{\pa... ...al q_k} \dot{q}_k + \frac{\partial^{ 2} x_j}{\partial q_i \partial t}\right)$$

$$= \dot{x}_j \frac{d}{dt}\!\left(\frac{\partial x_j}{\partial q_i}\right),$$ (B.16)

where use has been made of Equation (B.14). Thus, it follows from Equations (B.13), (B.15), and (B.16) that

$$\frac{d}{dt}\!\left(\frac{1}{2} \frac{\partial \dot{x}_j^{ 2}}{... ..._j}{\partial q_i} + \frac{1}{2} \frac{\partial \dot{x}_j^{ 2}}{\partial q_i}.$$ (B.17)

Let us take the previous equation, multiply by $m_j$ , and then sum over all $j$ . We obtain

$$\frac{d}{dt}\!\left(\frac{\partial K}{\partial \dot{q}_i}\right) ... ...l F}} f_j \frac{\partial x_j}{\partial q_i} + \frac{\partial K}{\partial q_i},$$ (B.18)

where use has been made of Equations (B.9) and (B.10). Thus, it follows from Equation (B.6) that

$$\frac{d}{dt}\!\left(\frac{\partial K}{\partial \dot{q}_i}\right) = Q_i + \frac{\partial K}{\partial q_i}.$$ (B.19)

Finally, making use of Equation (B.8), we get

$$\frac{d}{dt}\!\left(\frac{\partial K}{\partial \dot{q}_i}\right) = -\frac{\partial U}{\partial q_i}+\frac{\partial K}{\partial q_i}.$$ (B.20)

It is helpful to introduce a function $L$ , called the Lagrangian, which is defined as the difference between the kinetic and potential energies of the dynamical system under investigation:

$$L = K - U.$$ (B.21)

Because the potential energy, $U$ , is clearly independent of the $\dot{q}_i$ , it follows from Equation (B.20) that

$$\frac{d}{dt}\!\left(\frac{\partial L}{\partial \dot{q}_i}\right) -\frac{\partial L}{\partial q_i} =0,$$ (B.22)

for $i=1,{\cal F}$ . This equation is known as Lagrange's equation.

According to the previous analysis, if we can express the kinetic and potential energies of our dynamical system solely in terms of our generalized coordinates, and their time derivatives, then we can immediately write down the equations of motion of the system, expressed in terms of the generalized coordinates, using Lagrange's equation, (B.22). Unfortunately, this scheme only works for energy-conserving systems.