Definition
The total time derivative of the Lagrangian L is
dLdt=∑i∂L∂qidqidt+∑i∂L∂˙qid˙qidt+∂L∂t.
Since, by the Lagrange's equation,
∂L∂qi=ddt(∂L∂˙qi),
we have
dLdt=∑iddt(∂L∂˙qi)˙qi+∑j∂L∂˙qid˙qidt+∂L∂t=∑iddt(˙qi∂L∂˙qi)+∂L∂t.
So,
ddt(∑i˙qi∂L∂˙qi−L)+∂L∂t=0.
Define the Hamiltonian, or energy function, as
H(q1,...,qn,˙q1,...,˙qn,t)=∑i˙qi∂L∂˙qi−L.
Then, we have the equation
dHdt=−∂L∂t
Kinetic Energy
For a holonomic system,
→vi=d→ridt=∑j∂→ri∂qj˙qj+∂→ri∂tv2i=∑j∂→ri∂qj˙qj+∂→ri∂t⋅∑k∂→ri∂qk˙qk+∂→ri∂t=∑j∑k∂→ri∂qj⋅∂→ri∂qk˙qj˙qk+2∑k∂→ri∂qk⋅∂→ri∂t˙qk+∂→ri∂t⋅∂→ri∂t
Define
T0≡∑i12mi(∂→ri∂t)2,
T1≡∑j(∑imi∂→ri∂qj⋅∂→ri∂t)˙qj
and
T2≡12∑j,k(∑imi∂→ri∂qj⋅∂→ri∂qk)˙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(T2−V)=∂∂˙qλ[12∑j,k(∑imi∂→ri∂qj⋅∂→ri∂qk˙qj˙qk)]=12∑j,k(∑imi∂→ri∂qj⋅∂→ri∂qk)(δλi˙qk+˙qjδλk)=12∑k(∑imi∂→ri∂qλ⋅∂→ri∂qk)˙qk+12∑j(∑imi∂→ri∂qj⋅∂→ri∂qλ)˙qj=∑j(∑imi∂→ri∂qj⋅∂→ri∂qλ)˙qj.
The Hamiltonian is then
H≡∑λ∂L∂˙qλ˙qλ−L=∑λ,j(∑imi∂→ri∂qj⋅∂→ri∂qλ)˙qj˙qλ−(T2−V)=2T2−(T2−V)=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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