This is proof that L′ represents same equation of motion with L through Lagrange eq. I understand L′ satisfies Lagrange eq, but how does this proof mean L′ and L describe same motion of particle? In other words, why does total time derivative term which is added to L make no difference in equation of motion?
Answer
You have seen that the substitution L⟶L′:=L+dFdt
does not change the Euler-Lagrange equations. Now, this happens because the time derivative satisfies the Euler-Lagrange equations identically.
Let us consider a concrete example. Take the Lagrangian of a simple harmonic oscillator: LHO=12m˙q2−12mω2q2
which gives the Euler-Lagrange equations ¨q=−ω2q
Now consider the modified Lagrangian L′=12m˙q2−12mω2q2+˙q=LHO+˙q
The Euler-Lagrange equations are obviously linear. Thus EL[L′]=EL[LHO]+EL[˙q]
As was shown above, ˙q's Euler-Lagrange equation will vanish, but we can verify this: EL[˙q]:=ddt∂˙q∂˙q−∂˙q∂q=d1dt−0=0
Thus, EL[L′]=EL[LHO]
i.e. the modified Lagrangian still implies ¨q=−ω2q.
No comments:
Post a Comment