We saw before that Newton's 2nd law can be written in a more general form as
ddt∂L∂v(x,˙x)=∂L∂x(x,˙x),
known as the Euler-Lagrange equations. Hamilton discovered a principle that explains the origin of these equations. Consider some path of the system given by a curve γ, i.e.
x(t)=γ(t)
˙x(t)=ddtγ(t)
Then we may define a quantity associated with the path γ:
S=∫L(γ,˙γ)dt
called the action. Hamilton discovered the following.
Theorem The path taken by a mechanical system is one which extremizes the action.
To prove this, suppose we perturb the path a small amount, while leaving the endpoints fixed, i.e. γ↦γ+ϵ(δγ) with ϵ>0 small and δγ a path that is 0 at its endpoints. Then
L(γ+ϵδγ,˙γ+ϵδ˙γ)=L(γ,˙γ)+ϵ∂L∂xδγ+ϵ∂L∂vδ˙γ+o(ϵ2)
Thus
S[γ+ϵδγ]=S[γ]+ϵ∫∂L∂xδγdt+ϵ∫∂L∂vδ˙γdt+o(ϵ2)
Integrating by parts, and using the fact that δγ is 0 on the endpoints, we have
∫∂L∂vδ˙γdt=−∫ddt∂L∂vδγdt
Combining the above, we have
δSδγ(δγ)=∫(∂L∂x−ddt∂L∂v)δγdt
Thus the variational derivative of S is
δSδγ=∂L∂x−ddt∂L∂v
So a path γ is a critical point of S (i.e. it extremizes S) if and only if the Euler-Lagrange equations are satisfied.
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