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Happiness
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Why isn't ##\frac{\partial L}{\partial t}\frac{\partial t}{\partial \dot{q_m}}## included in (5.41), given that ##L## could depend on ##t## explicitly?
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Orodruin said:##t## does not depend on the coordinates.
"##t## does not depend on the coordinates" ## \Leftrightarrow (\frac{\partial x}{\partial t})_{y,z}=0##!Happiness said:Suppose ##(\frac{\partial x}{\partial t})_{y,z}\neq0##. Doesn't that imply ##(\frac{\partial t}{\partial x})_{y,z}=((\frac{\partial x}{\partial t})_{y,z})^{-1}\neq0##?
Shayan.J said:"##t## does not depend on the coordinates" ## \Leftrightarrow (\frac{\partial x}{\partial t})_{y,z}=0##!
You are considering a transformation taking x to q and t to t. Regardless of what transformation you have done, t only depends on t.Happiness said:Suppose a particle travels at constant velocity, i.e., ##x=vt##. Then ##\frac{\partial x}{\partial t}=v\neq0##. So does ##t## depends on ##x##? I'm confused.
No, t still does not depend on the coordinates.Happiness said:In that case, should ∂L∂t∂t∂˙qm∂L∂t∂t∂qm˙\frac{\partial L}{\partial t}\frac{\partial t}{\partial\dot{q_m}} be included in (5.41)?
Orodruin said:No, t still does not depend on the coordinates.
vanhees71 said:I don't know what Eq. (5.41) is, but you should get the form-invariant Euler-Lagrange equations also in the new coordinates, i.e., you have given
$$L'(q,\dot{q},t)=L[x(q,t);\dot{x}(q,t),t].$$
where
$$\dot{x}^i=\dot{q}^k \frac{\partial x^i}{\partial q^k}+\partial_t x^i.$$
In this equation the ##\partial_t x^i## means the derivative of the explicit time dependence of ##x^i(q,t)##.
Then the equations of motion in the new coordinates read
$$\frac{\partial L'}{\partial q^k}-\frac{\mathrm{d}}{\mathrm{d} t} \frac{\partial L'}{\partial \dot{q}^k}=0.$$
In general, for partial derivatives, ##\partial x^i/\partial y^j## is not equal to the reciprocal of ##\partial y^j/\partial x^i##. You need to consider what these functions actually are functions of and what variable change you are considering. In this case you are keeping t as an independent variable and its parial derivative wrt anything else is zero.Happiness said:Does that mean ##\frac{\partial t}{\partial\dot{q_m}}=0## and ##\frac{\partial t}{\partial q_m}=0##? Why?
Clearly, ##\frac{\partial q_m}{\partial t}\neq0##.
The Euler-Lagrange equation is a mathematical formula used in the field of calculus of variations. It is used to find the function that minimizes or maximizes a given integral, subject to certain constraints.
The Euler-Lagrange equation is derived using the calculus of variations, a branch of mathematics that deals with finding the extrema of functionals. It involves setting up a functional and then finding the function that minimizes or maximizes it using the Euler-Lagrange equation.
The change of coordinates allows us to transform the original problem into a simpler one, making it easier to find the function that minimizes or maximizes the functional. This is particularly useful when dealing with non-Cartesian coordinate systems, where the equations may be more complex.
The derivation of the Euler-Lagrange equation assumes that the functional is continuous, has continuous first-order derivatives, and has a unique solution. It also assumes that the boundary conditions are fixed and do not vary with the function.
The Euler-Lagrange equation has many applications in physics, engineering, and economics. It is used to find the shortest path between two points, the shape of a hanging chain, and the optimal trajectory for a rocket. It is also used in the study of optimal control and game theory.