Hamiltonian

Definition

The total time derivative of the Lagrangian L is dLdt=iLqidqidt+iL˙qid˙qidt+Lt. Since, by the Lagrange's equation, Lqi=ddt(L˙qi), we have dLdt=iddt(L˙qi)˙qi+jL˙qid˙qidt+Lt=iddt(˙qiL˙qi)+Lt. So, ddt(i˙qiL˙qiL)+Lt=0. Define the Hamiltonian, or energy function, as H(q1,...,qn,˙q1,...,˙qn,t)=i˙qiL˙qiL. Then, we have the equation dHdt=Lt

Kinetic Energy

For a holonomic system, vi=dridt=jriqj˙qj+ritv2i=jriqj˙qj+ritkriqk˙qk+rit=jkriqjriqk˙qj˙qk+2kriqkrit˙qk+ritrit Define T0i12mi(rit)2, T1j(imiriqjrit)˙qj and T212j,k(imiriqjriqk)˙qj˙qk Thus, the total kinetic energy can be divided into three terms T=i12miv2i=T0+T1+T2 If the constraint is time-independent, then the transformation ri=ri(q1,...,qN) is also time-independent, then T0=T1=0

Hamiltonian VS total energy

If the potential energy V is velocity independent V=V(q1,...,qN,t) and constraint is time-independent, i.e. T=T2, then L˙qλ=˙qλ(T2V)=˙qλ[12j,k(imiriqjriqk˙qj˙qk)]=12j,k(imiriqjriqk)(δλi˙qk+˙qjδλk)=12k(imiriqλriqk)˙qk+12j(imiriqjriqλ)˙qj=j(imiriqjriqλ)˙qj. The Hamiltonian is then HλL˙qλ˙qλL=λ,j(imiriqjriqλ)˙qj˙qλ(T2V)=2T2(T2V)=T2+V=T+V, which is the total energy. Therefore, Hamiltonian is equivalent to the total energy on the condition that potnetial energy does not depend on generalized velocity and constraint does not depend on time.

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