# Lagrangian for free particle

1. Jun 11, 2008

### rc75

In section 4 of Landau and Lifgarbagez they derive the expression for the kinetic energy by expanding the Lagrangian around v+e. The resulting expression has a term which must be a total time derivative so that the equations of motion are unaffected. The text claims that the term dL/d(v^2) v.e must be linear in v to be a total time derivative, but I don't understand why this is.

2. Jun 17, 2008

### dslowik

I just read that section.
I think it would have helped if they stated that the 2nd term is a total time derivative 'of a function of coordinates and time' ...
df(x,t) / dt = df/dx * dx/dt + df/dt (partial d's now)
Since f does not depend on the velocities, df/dx and df/dt don't, and the overall dependence of df/dt on v=dx/dt is linear.

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