# Lagrangian and equations of motion in a time-varying coordinate system

I am assuming a very simple case, where there is only a mass $$m$$ with position $$x$$ under an external force $$F$$. we know that the Lagrangian takes the form $$L = (1/2) m \dot{x}^2$$ from which equations of motion follow as $$\frac{d}{d t} \frac{\partial L}{\partial \dot{x}}= m \ddot{x} = F\tag{1}$$ respectively.

Now, consider a coordinate transformation $$y(t) = \frac{x(t)}{n(t)} + \int_0^t\frac{x(\tau)\dot{n}(\tau)}{n^2(\tau)} d\tau\tag{2}$$ which yields $$\dot{y}(t)=\frac{\dot{x}(t)}{n(t)}.\tag{3}$$ I am wondering what is wrong with the following derivation if I want to find the equations of motion through Euler-Lagrange equation in this new coordinate system:

• The external force can be mapped to new coordinate system by the equivalence of virtual work $$F_\text{new} = n(t) F$$.

• The Lagrangian can be expressed in the new frame as $$L=(1/2) m (\dot{y}(t) n(t))^2$$.

• Therefore, the EL equation is obtained as $$\frac{d}{d t} \frac{\partial L}{\partial \dot{y}} =m n^2(t) \ddot{y}(t)+ 2m \dot{n}(t) n(t) \dot{y}(t)= F_\text{new},\tag{4}$$ which doesn't seem correct to me, since a simple substitution into the equation of motion directly gives $$m n^2(t) \ddot{y} + m \dot{n} n(t) \dot{y}(t) = F_\text{new}\tag{5}$$ instead.

1. Let $$J(t)~:=~\int_{t_i}^t \!dt^{\prime}~F(t^{\prime}) \tag{1}$$ be the external impulse.

2. OP's system has Lagrangian$$^1$$ $$L(x,\dot{x},t) ~=~ \frac{m}{2}\dot{x}^2 +Fx\qquad\stackrel{\text{Appropriate BCs} }{\Rightarrow}\qquad m\ddot{x}~\approx~F.\tag{2}$$

3. OP's non-local transformation reads $$y(t)~:=~\frac{x(t)}{n(t)} + \int_{t_i}^t \!dt^{\prime} \frac{\dot{n}(t^{\prime}) x(t^{\prime})}{n(t^{\prime})^2}\quad\Leftrightarrow\quad \dot{x}~=~n\dot{y}\quad\Leftrightarrow\quad x(t)~=~n(t)y(t)- \int_{t_i}^t \!dt^{\prime}~\dot{n}(t^{\prime}) y(t^{\prime}).\tag{3}$$

4. We can in principle rewrite the action $$\int_{t_i}^{t_f}\!dt~ L(x,\dot{x},t)~=~S~~=~\int_{t_i}^{t_f}\!dt~\widetilde{L}(y,\dot{y},t)\tag{4}$$ in the new $$y$$-variable. The result is $$\widetilde{L}(y,\dot{y},t) ~=~\frac{m}{2}n^2\dot{y}^2 +\frac{d(n(J\!-\!J(t_f))}{dt}y\qquad\stackrel{\text{Appropriate BCs} }{\Rightarrow}\qquad \frac{d(mn^2\dot{y})}{dt}~\approx~\frac{d(n(J\!-\!J(t_f)))}{dt}.\tag{5}$$

5. Let us now return to OP's question. The main issue with OP's construction is that the non-local transformation (3) does not necessarily take appropriate BCs for $$x$$ into appropriate BCs for $$y$$. However, the 2 EL equations (2) & (5) do agree if we pick the BCs $$x(t_i)~=~x_i\qquad\text{and}\qquad \dot{x}(t_f)~=~0.\tag{6}$$

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$$^1$$ Recall that the derivation of Euler-Lagrange (EL) equations from an stationary action principle is only possible with appropriate boundary conditions (BCs).

• thanks. I haven't been able to understand it yet. what do you mean by appropriate boundary condition? can you give an example to non-appropriate BCs? – GSecer Jun 13 '19 at 13:11
• I updated the answer. – Qmechanic Jun 13 '19 at 13:11