- #1
davidbenari
- 466
- 18
I was solving the double pendulum problem via Lagrangian methods but something bothered me quite a lot.
(Consider the two bobs are of equal mass and the pendula are of equal length). Then the potential energies are conveniently written as ##V_1=-mgl\cos\theta## and ##V_2=-mg(l\cos\theta+l\cos\phi)##. Where ##\big\{\theta,\phi\big\}## refer to the usual coordinate angles. ##V## total would be ##V=V_1+V_2##.
What bothers me is:
Why isn't the potential energy written (analogously to EM) as ##\frac{1}{2}\sum m_i \Phi(\mathbf{r})## where ##\Phi## would be the gravitational voltage (and of course this includes the sun too).
As another example the potential energy of a baseball is written in the Lagrangian formalism as ##-mgy## where ##y## is the height. Why is it valid to say that a single particle can have potential energy?
Namely, potential energy is strictly about a system of particles. We sometimes (incorrectly) speak of the potential energy of a particle only when we're interested in the dynamics of the particle and know that we can find the force on it via the potential energy.
So I arrived to the conclusion that the potential energy in Lagrangian mechanics more precisely refers to that quantity which would give the correct generalized force; not the actual potential energy of the system. In that sense Lagrangian mechanics isn't about potential energies but about the scalar potential in vector calculus. Which although related, aren't the same thing.
Where am I screwing up here?
Thanks.
(Consider the two bobs are of equal mass and the pendula are of equal length). Then the potential energies are conveniently written as ##V_1=-mgl\cos\theta## and ##V_2=-mg(l\cos\theta+l\cos\phi)##. Where ##\big\{\theta,\phi\big\}## refer to the usual coordinate angles. ##V## total would be ##V=V_1+V_2##.
What bothers me is:
Why isn't the potential energy written (analogously to EM) as ##\frac{1}{2}\sum m_i \Phi(\mathbf{r})## where ##\Phi## would be the gravitational voltage (and of course this includes the sun too).
As another example the potential energy of a baseball is written in the Lagrangian formalism as ##-mgy## where ##y## is the height. Why is it valid to say that a single particle can have potential energy?
Namely, potential energy is strictly about a system of particles. We sometimes (incorrectly) speak of the potential energy of a particle only when we're interested in the dynamics of the particle and know that we can find the force on it via the potential energy.
So I arrived to the conclusion that the potential energy in Lagrangian mechanics more precisely refers to that quantity which would give the correct generalized force; not the actual potential energy of the system. In that sense Lagrangian mechanics isn't about potential energies but about the scalar potential in vector calculus. Which although related, aren't the same thing.
Where am I screwing up here?
Thanks.