- #1
- 5,199
- 38
In describing the motion (orbit) of a particle of mass m moving in a plane in a potential V(r), my prof wrote the Lagrangian as:
[tex] \mathcal{L} = \frac{1}{2}m|\dot{\vec{r}}|^2 - V(r) [/tex]
Without further ado (or explanation of any kind), he wrote this as:
[tex] \mathcal{L} = \frac{1}{2}m(\dot{r}^2 + r^2\dot{\phi}^2) - V(r) [/tex]
Where [itex] (r,\phi) [/itex] are the coordinates in the orbital plane.
I understand that he is expressing the magnitude of r'(t) in terms of its components, and that [itex] \dot{r} [/itex] is the velocity in the radial direction, whereas the second term should represent tangential velocity. But that part bothers me. After all...how did he arrive at this second term? It seems to me that he assumed r was constant, otherwise the second term is invalid. Yet if r were constant, then [itex] \dot{r} [/itex] would be zero!
Shouldn't it be done as follows?
[tex] |\dot{\vec{r}}|^2 = \left(\frac{ds}{dt}\right)^2 = \left(\frac{d(r \phi)}{dt}\right)^2 = (\dot{r}\phi + r\dot{\phi})^2 [/tex]
What am I doing wrong, and how did he get his formula?
[tex] \mathcal{L} = \frac{1}{2}m|\dot{\vec{r}}|^2 - V(r) [/tex]
Without further ado (or explanation of any kind), he wrote this as:
[tex] \mathcal{L} = \frac{1}{2}m(\dot{r}^2 + r^2\dot{\phi}^2) - V(r) [/tex]
Where [itex] (r,\phi) [/itex] are the coordinates in the orbital plane.
I understand that he is expressing the magnitude of r'(t) in terms of its components, and that [itex] \dot{r} [/itex] is the velocity in the radial direction, whereas the second term should represent tangential velocity. But that part bothers me. After all...how did he arrive at this second term? It seems to me that he assumed r was constant, otherwise the second term is invalid. Yet if r were constant, then [itex] \dot{r} [/itex] would be zero!
Shouldn't it be done as follows?
[tex] |\dot{\vec{r}}|^2 = \left(\frac{ds}{dt}\right)^2 = \left(\frac{d(r \phi)}{dt}\right)^2 = (\dot{r}\phi + r\dot{\phi})^2 [/tex]
What am I doing wrong, and how did he get his formula?