4. Consider a harmonic oscillator in two dimensions described by the Lagrangian m mw? L = "( +r*ở*) - 2 where (r, o) are the polar coordinates, m > 0 is the mass, and w > 0 the oscillation frequency of the oscillator. (a) Find the canonical momenta p, and po, and show that the Hamiltonian is given by mw? H(r, ø, Pr: Po) 2m' 2mr2 2 (b) Show that p, is a constant of motion. (c) In the following we will perform a variable transformation (r, 6, pr; Ps) → (Qr, Qó, Pr, Po defined by Q, = lp, P, Po Po = 2 Qo = 20, 2r where l is an arbitrary constant length that just ensures that Q, has also the dimension of length. Show that this transformation is canonical.

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4. Consider a harmonic oscillator in two dimensions described by the Lagrangian
т
mw?
L
2
2
where (r, ø) are the polar coordinates, m > 0 is the mass, and w > 0 the oscillation
frequency of the oscillator.
(a) Find the canonical momenta p, and pø, and show that the Hamiltonian is given by
mw?
H(r, 0, Pr; Po) =
2m
2mr?
2
(b) Show that p is a constant of motion.
(c) In the following we will perform a variable transformation (r, 6, p,, Pa) → (Q,,Qo, Pr, Pg
defined by
Q, = 7.
lp,
P,
Pg = Pe
Qo = 20,
%3D
%3D
2r
2
where l is an arbitrary constant length that just ensures that Q, has also the dimension
of length. Show that this transformation is canonical.
(d) The Hamiltonian becomes in the new variables
2P?
mw?l
2
H(Q..Qo» Pr, Pa) = Q,P; +
mlQ,
-Qr.
2
ml
Show that the fact that energy E is conserved and H = E allows us to introduce a new
Hamiltonian HK and a new energy EK such that HK
with
%3D
Ek is equivalent to H = E,
P2
P?
Hg(Qr.Qá, Pr, Pa) =
|
2m
2mQ? Q,
Determine the constant a and the new energy EK as a function of m,w, E.
(e) In question 4 (d) just above you will find that one always has EK < 0 and a > 0. Give
a physical argument why this must indeed be the case.
Transcribed Image Text:4. Consider a harmonic oscillator in two dimensions described by the Lagrangian т mw? L 2 2 where (r, ø) are the polar coordinates, m > 0 is the mass, and w > 0 the oscillation frequency of the oscillator. (a) Find the canonical momenta p, and pø, and show that the Hamiltonian is given by mw? H(r, 0, Pr; Po) = 2m 2mr? 2 (b) Show that p is a constant of motion. (c) In the following we will perform a variable transformation (r, 6, p,, Pa) → (Q,,Qo, Pr, Pg defined by Q, = 7. lp, P, Pg = Pe Qo = 20, %3D %3D 2r 2 where l is an arbitrary constant length that just ensures that Q, has also the dimension of length. Show that this transformation is canonical. (d) The Hamiltonian becomes in the new variables 2P? mw?l 2 H(Q..Qo» Pr, Pa) = Q,P; + mlQ, -Qr. 2 ml Show that the fact that energy E is conserved and H = E allows us to introduce a new Hamiltonian HK and a new energy EK such that HK with %3D Ek is equivalent to H = E, P2 P? Hg(Qr.Qá, Pr, Pa) = | 2m 2mQ? Q, Determine the constant a and the new energy EK as a function of m,w, E. (e) In question 4 (d) just above you will find that one always has EK < 0 and a > 0. Give a physical argument why this must indeed be the case.
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