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Classical Mechanics: Difference between revisions
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Computed in the particular example of a Lagrangian given previously. Regarding the position and velocity variables as functions of time again means that given a particular formula for a Lagrangian, the resulting Euler-Lagrange equation has the form of an ordinary differential equation whose solution is just the function <math> q(t) </math>. The partial derivatives of L with respect to position and velocity at each time indicate vectors in the constant-time planes as in the picture, that point in the direction of increase of L. Thus the Euler-Lagrange equation for motion in one dimension can be interpreted in the 3d geometric picture. The change in the velocity component of this vector as one moves up in time along the trajectory (e.g. it decreases) is the instantaneous value of the position component. This time derivative is in the case of when L is time independent, totally determined by the direction of the trajectory in the 3d graphed trajectory space at that time. Given an initial position and velocity at some time, this fixes the rest of the curve for future times, as it is constrained to follow the direction given by the EL equation at each time. In particular, this gives the endpoint at some chosen later time. | Computed in the particular example of a Lagrangian given previously. Regarding the position and velocity variables as functions of time again means that given a particular formula for a Lagrangian, the resulting Euler-Lagrange equation has the form of an ordinary differential equation whose solution is just the function <math> q(t) </math>. The partial derivatives of L with respect to position and velocity at each time indicate vectors in the constant-time planes as in the picture, that point in the direction of increase of L. Thus the Euler-Lagrange equation for motion in one dimension can be interpreted in the 3d geometric picture. The change in the velocity component of this vector as one moves up in time along the trajectory (e.g. it decreases) is the instantaneous value of the position component. This time derivative is in the case of when L is time independent, totally determined by the direction of the trajectory in the 3d graphed trajectory space at that time. Given an initial position and velocity at some time, this fixes the rest of the curve for future times, as it is constrained to follow the direction given by the EL equation at each time. In particular, this gives the endpoint at some chosen later time. | ||
For the Lagrangian given, at initial condition <math> (1, 0, 0) </math> the solution is <math> cos(\sqrt{\frac{k}{m}}t) </math> which is the familiar oscillator of mass m and spring constant k. For higher dimensional and multi-particle systems, the generalization is from considering one EL equation to one for each dimension of position for each particle. For N particles in three dimensions, this gives 3N equations. Often, the Lagrangian is divided into two terms <math> L=T(\dot{q}_1, \dot{q}_2, \cdots, \dot{q}_{3N})-U(q_1, q_2, \cdots, q_{3N}) </math>, with the term T depending only on the velocities is called the kinetic energy and U the potential energy. Now, we have an opportunity to analyze these terms and equations more generally. The derivatives <math> \frac{\partial L}{\partial \dot{q}_i} = \frac{\partial T}{\partial \dot{q}_i}, \quad i = 1, 2, \cdots, 3N</math> assembled as a vector are known as the momentum of the system. This another reason Lagrangians are powerful. Letting the <math> q_i </math> coordinates be coordinates other than Cartesian, e.g. spherical coordinates for each particle, allows us to discuss linear and angular momentum on the same footing or momentum in any convenient coordinate system. Given <math> m_{1+((i-1)-(i-1)\,mod 3)/3} = m_k </math> as the mass of the k'th particle in order, in cartesian coordinates <math> T=\sum_{i=1}^{3N} \frac{1}{2}m_k \dot{q}^2_i </math> gives <math> \frac{\partial T}{\partial \dot{q}_i}=m_k \dot{q}_i=m_k v_i </math>. Similarly, the coordinate derivative gives <math> \frac{\partial L}{\partial q_i}=-\frac{\partial U}{\partial q_i} </math> which has the interpretation of force on the i'th coordinate. Then the i'th EL equation is expressed as <math> m_k \frac{d}{dt} v_i = m_k a_i= F_i </math> which is just Newton's second law. Indeed, all of Newton's laws can be derived from this formulation of motion, but to do so fully we need an equation using finite properties of the Lagrangian and not just an infinitesimal condition. | For the Lagrangian given, at initial condition <math> (1, 0, 0) </math> the solution is <math> cos(\sqrt{\frac{k}{m}}t) </math> which is the familiar oscillator of mass m and spring constant k. For higher dimensional and multi-particle systems, the generalization is from considering one EL equation to one for each dimension of position for each particle. For N particles in three dimensions, this gives 3N equations. Often, the Lagrangian is divided into two terms <math> L=T(\dot{q}_1, \dot{q}_2, \cdots, \dot{q}_{3N})-U(q_1, q_2, \cdots, q_{3N}) </math>, with the term T depending only on the velocities is called the kinetic energy and U the potential energy. Now, we have an opportunity to analyze these terms and equations more generally. The derivatives <math> \frac{\partial L}{\partial \dot{q}_i} = \frac{\partial T}{\partial \dot{q}_i}, \quad i = 1, 2, \cdots, 3N</math> assembled as a vector are known as the momentum of the system. This another reason Lagrangians are powerful. Letting the <math> q_i </math> coordinates be coordinates other than Cartesian, e.g. spherical coordinates for each particle, allows us to discuss linear and angular momentum on the same footing or momentum in any convenient coordinate system. Given <math> m_{1+((i-1)-(i-1)\,mod 3)/3} = m_k </math> as the mass of the k'th particle in order, in cartesian coordinates <math> T=\sum_{i=1}^{3N} \frac{1}{2}m_k \dot{q}^2_i </math> gives <math> \frac{\partial T}{\partial \dot{q}_i}=m_k \dot{q}_i=m_k v_i </math>. Similarly, the coordinate derivative gives <math> \frac{\partial L}{\partial q_i}=-\frac{\partial U}{\partial q_i} = F_i </math> which has the interpretation of force on the i'th coordinate. Then the i'th EL equation is expressed as <math> m_k \frac{d}{dt} v_i = m_k a_i= F_i </math> which is just Newton's second law. Indeed, all of Newton's laws can be derived from this formulation of motion, but to do so fully we need an equation using finite properties of the Lagrangian and not just an infinitesimal condition. | ||