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Transcript
Advanced Mechanics 241, Spring 2008
Examination Questions and Problems
Part I. Questions
1. Formulate Hamilton’s principle (the principle of least action) and use it to derive Lagrange’s
equations of motion for a system with s degrees of freedom. What is called the number of
degrees of freedom? What are the properties of the Lagrangian of a mechanical system? What
is the Lagrangian for a free particle in relativistic mechanics?
2. Formulate Noether’s theorem and use it to derive the conservation laws of energy, momentum
and angular momentum of a classical mechanical system with s degrees of freedom. What
symmetries are responsible for the existence of the conservation laws?
3. Discuss the motion in one dimension and in a central field. What is a turning point? What
is a potential well? If the motion is finite how can one find the period of oscillations. How do
you find the equation of the path of a particle in a central field. What types of paths exist?
4. Discuss Kepler’s problem in an attractive field. What is the Runge-Lenz vector?
5. How do you find the deflection angle of a particle scattered by a central field? What is the
impact parameter? Find the deflection angle of a particle moving in a repulsive Coulomb field.
6. Discuss small oscillations of a system with s degrees of freedom. What is the characteristic
equation? What are the characteristic (or normal, or eigen) frequencies of the system? What
are normal coordinates?
7. Discuss forced oscillations of a one-dimensional harmonic oscillator if the external force is
F (t) = f cos(γt + β) .
What is resonance?
8. Discuss damped oscillations of a one-dimensional harmonic oscillator.
9. Discuss forced oscillations of a one-dimensional harmonic oscillator under friction if the
external force is
F (t) = f cos(γt + β) .
10. Express the kinetic energy of a rigid body as a sum of the kinetic energy of the translational
motion and the kinetic energy of the rotation. Define the inertia tensor. What are principal
axes of inertia? What are principal moments of inertia? What is a symmetrical top? What is
a spherical top?
11. Derive the expression for the angular momentum of a rigid body in terms of its inertia
tensor and its angular velocity. Define the inertia tensor. What are principal axes of inertia?
What are principal moments of inertia? What is a symmetrical top? What is a spherical top?
Discuss the free motion of a symmetric top. What is the regular precession of a symmetric top?
12. Determine the frequency of small oscillations of a compound pendulum (a rigid body
swinging about a fixed horizontal axis in a gravitational field).
13. Derive the equations of motion of a rigid body in a fixed system of reference. What is the
total torque?
1
14. What are the Euler angles? What is the line of nodes? Represent a general rotation matrix
as a product of three simple rotation matrices. Express the components of the angular velocity
vector Ω along the moving axes x1 , x2 , x3 in terms of the Eulers angles and their derivatives.
Determine the free motion of a symmetric top by using the Euler angles.
15. Integrate (reduce to quadratures) the equations of the motion of a heavy symmetrical top
whose lowest point is fixed (Lagrange’s top). Use the Euler angles. What angles are cyclic?
What is nutation? What types of motion are possible?
15. Derive Euler’s equations, that is the equations of motion of a rigid body in a moving system
of reference. Use them to analyze the free motion of a symmetrical top.
16. This problem will not be given this year. Find the Lagrangian describing the motion
of a particle in an arbitrary non-inertial frame of reference. What is the Coriolis force? What
is the centrifugal force? What is centrifugal energy?
17. Discuss Legendre’s transformation, and apply it to a system with the following Lagrangian
1
L = gij (q) q̇ i q̇ j − U (q) .
2
18. Define the Poisson bracket of two functions on the phase space and discuss its properties.
Formulate and prove Poisson’s theorem.
19. Define canonical transformations. How can the generating functions Si be used? Show that
canonical transformations preserve the Poisson brackets. How can a function V (p, q) be used
to generate a canonical transformation?
20. Discuss Liouville’s theorem.
21. Discuss the action as a function of the coordinates and time, and show that it is a generating
function of a canonical transformation.
22. Discuss the Hamilton-Jacobi equation and separation of variables. When can the spherical
coordinates be used to separate variables? Use the Hamilton-Jacobi equation to analyze the
motion of a harmonic oscillator.
23. Discuss adiabatic invariants. Find the adiabatic invariant for a harmonic oscillator.
24. Discuss action/angle variables. Find action-angle variables I and α for a harmonic oscillator
as functions of p and x. Find p and x as functions of action-angle variables I and α for a
harmonic oscillator.
25. Discuss conditionally periodic motion.
26. What is the principle of relativity of Einstein? Define the interval between two events.
What is a timelike interval? What is a spacelike interval? What is a lightlike interval? What
is the proper time?
27. Derive the Lorentz transformation in the tx-plane. What is the proper length? What are
Poincare transformations? Derive the law of transformation of velocities.
28. Define covariant and contravariant four-vectors. What is the metric tensor of the 4dimensional Minkowski space-time? Define four-dimensional tensors of rank 2 and 3. What
is the trace of a second-rank tensor? What is the operation of contraction? What is the
2
completely antisymmetric unit pseudotensor of fourth rank? What is a pseudotensor of any
rank?
29. What is the action of a free relativistic particle? What are the equations of motion of a
relativistic particle if an external force F acts on it? What is the momentum of a free relativistic
particle? What is the energy of a free relativistic particle? How are they related? What is
the momentum four-vector? What is the angular momentum four-tensor of a free relativistic
particle?
30. Consider the problem of a vibrating elastic rod by first discretizing the system, and then
by taking the continuum limit. Derive the Lagrangian of the resulting continuous system and
the equation of motion. Write the general solution to the equation of motion and explain its
meaning.
31. Discuss the Lagrangian formulation for continuous systems. What are Dirichlet boundary
conditions?
32. Discuss the stress-energy tensor and conservation laws.
33. Discuss the Hamiltonian formulation for continuous systems.
34. Discuss the relativistic field theory of a complex scalar field. What is the Lagrangian
density? What are the equations of motion? What is the Hamiltonian density? What is the
momentum representation? What is the U (1) symmetry, the corresponding conserved U (1)
current and charge?
35. Discuss the relativistic field theory of an electromagnetic field. What is the Lagrangian
density? What are the equations of motion? What is the gauge symmetry? How does a point
particle interact with the electromagnetic field? What is the Lagrangian?
Part II. Problems.
1. Find the Lagrangian of a particle moving on the surface of a paraboloid z = a(x2 + y 2 ) in a
uniform gravitational field F = {Fx , Fy , Fz }. What is the number of degrees of freedom of this
system?
2. Use Noether’s theorem to find conserved charges, Jij , corresponding to the rotational symmetry of the Lagrangian of a particle moving in a D-dimensional flat space
r
q
vi2 κ
2
L = −mc 1 − 2 − , r = x2i .
c
r
How many independent charges are there? Compute the Poisson brackets of the charges with
the momenta pk , coordinates xk and themselves, that is
{Jij , pk } ,
{Jij , xk } ,
{Jij , Jkl } .
Show that for any choice of the indices i, j, k, l the Poisson bracket {Jij , Jkl } can be written
as a linear combination of the charges. Specify the formulas for the D = 3 case. What is the
relation of Jij for D = 3 with the angular momentum M?
3. Find the one-dimensional particle motion in the trigonometric potential
U (x) = V tan2 (a x) ,
3
V > 0.
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4. Integrate (reduce to quadrature) the equations of motion for a particle moving on the surface
of a paraboloid z = a(x2 + y 2 ) in a uniform gravitational field F = {0, 0, Fz }.
5. Integrate the equations of motion for a particle in a central field
U =−
α
,
r2
4
α>0
6. Find the frequency of oscillations of a particle of mass m which is free to move along the
parabola y = −ax2 and is attached to an ideal spring whose other end is fixed at a point A
with coordinates (0, l). A force F is required to extend the spring to length l.
7. Determine the forced oscillations and find the energy acquired by an oscillator experiencing
a force
F0 eαt
t<0
F (t) =
−αt
F0 e
t>0
The initial energy as t → −∞ is E0 = 0.
8. Find the normal coordinates and frequencies of a system of three equal masses constrained
to move on a circle connected to each other by identical ideal springs.
9. Determine the motion of an oscillator due to an external force
F0 eαt
t<0
F (t) =
F0 e−αt
t>0
in the presence of friction.
10. Find the principal moments of inertia for the following homogeneous bodies: (a) a thin
rod of length l, (b) a sphere of radius R, (c) a circular cylinder of raius R and height h, (d) a
rectangular parallelepiped of sides a, b, and c, (e) a circular cone of height h and base radius
R, (f ) an ellipsoid of semiaxes a, b, and c.
Find the frequency of small oscillations of a rigid body (for each of them) about a horizontal
axis passing through the point of the body farthest from the centre of mass and perpendicular
to the line connecting the point and the centre of mass.
11. Find the principal moments of inertia of a homogeneous ball of radius R which has a cavity
in the shape of a sphere of radius r. The distance between the centres of the ball and the sphere
is a ≤ R − r.
Find the frequency of small oscillations of the rigid body about a horizontal axis perpendicular to the line connecting the centres of the ball and the sphere and passing through the
center of
(i) the ball.
(ii) the cavity.
12. A symmetric top with a fixed center of mass experiences a constant torque K. Use the
Euler angles to find the angular velocity of the top as a function of time. The initial angular
velocity is equal to 0: Ω(0) = 0.
13. A heavy symmetrical top whose lowest point is fixed rotates about a vertical axis. Find
(i) The condition for the rotation to be stable.
(ii) The frequency of small oscillations in the θ-direction if the top was shifted from the
equilibrium position.
14. Find the Hamiltonian and Hamilton’s equations of motion for a system with two degrees
of freedom with the following Lagrangian
L=
m1 2 m2 2
ẋ +
ẋ + B(ẋ1 x2 − x1 ẋ2 ) − U (x1 , x2 ) .
2 1
2 2
15. Find the Hamiltonian and Hamilton’s equations of motion for a free relativistic particle of
rest mass m0 (c is the speed of light)
r
1
2
L = −m0 c 1 − 2 ẋ2i .
c
16. Let S1 (q, Q, t), S2 (q, P, t), S3 (p, Q, t), S4 (p, P, t) be the four generating functions of a
canonical transformation. Express
1. p and P , and H 0 − H as the derivatives of S1 (q, Q, t).
2. p and Q, and H 0 − H as the derivatives of S2 (q, P, t).
3. q and P , and H 0 − H as the derivatives of S3 (p, Q, t).
4. q and Q, and H 0 − H as the derivatives of S4 (p, P, t).
17. Use the four generating functions S1 (q, Q, t), S2 (q, P, t), S3 (p, Q, t), S4 (p, P, t) to prove that
{Qi , Qj }p,q = 0 ,
{Pi , Pj }p,q = 0 .
18. Find the explicit form (i.e. P = P (p, q) , Q = Q(p, q)) of the canonical transformation
generated by the function
V (p, q) = λ pi q i ,
λ is a constant ,
by means of the formula
1
f (p, q) → f (P, Q) = f (p, q) + {V (p, q), f (p, q)} + {V, {V, f }} + · · ·
2
∞
X
1
= f+
{V, {V, · · · { V, f } · · · } ,
{z
}
n! |
n=1
n
where f (p, q) is an arbitrary function of the phase space.
19. Find canonical transformations corresponding to the generating functions below and explain
their meaning
5
1. S3 (p, Q) = pi Qi − ai pi , for some fixed vector ai .
2. S2 (q, P ) = −q i Pi − ijk ai q j Pk , summation over i, j, k..
3. S3 (p, Q, t) = pi Qi + τ H(p, Q, t).
2
4. S1 (q, Q) = m2 (Q−q)
, where m, t1 , t2 are constants.
t2 −t1
Consider τ and ai to be infinitesimally small, i.e. neglect the terms containing τ 2 etc.
20. The transformations below are almost canonical. Find a simple way to modify them so
that they become canonical.
1.
(q+p)4
(q+p)4
P = p + q , Q = 4q e
+ 1 + p 4e
−1 .
2.
P = qp ,
Q = ln p3007 q 2006 .
21. Find generating functions S2 (q, P, t) producing the following canonical transformations of
the six-dimensional phase space
1. Spiral motion.
22. Using the Hamilton-Jacobi equation find the trajectory and the motion of a particle in the
potential
U (r) = −F x .
Note that it is a three-dimensional problem.
23. Find action-angle variables I and α for a harmonic oscillator as functions of p and x. Find
p and x as functions of action-angle variables I and α for a harmonic oscillator.
24. Prove that in the theory of relativity the sum of two velocities each smaller than the velocity
of light is again not greater than the light velocity.
25. The electric, E, and magnetic, B, field strengths are components of the electromagnetic
field tensor Fij


0
Ex
Ey
Ez
 −Ex
0 −Bz By 
.
Fij = 
 −Ey Bz
0
−Bx 
−Ez −By Bx
0
Find the law of transformation of the components of E and B under Lorentz transformations
in the x0 x1 -plane.
26. Find the Hamiltonian and Hamilton’s equations of motion for a charge e moving in a
constant uniform electric field E described by the following action
!
r
Z t2
2
v
S=
dt −mc2 1 − 2 + e r·E .
c
t1
Then, take the direction of the field as the x-axis, and the momentum at t = 0 to be p0 =
(0, p0 , 0). Obtain the equation of the trajectory, and sketch its plot. Find the trajectory if the
velocity of the particle is v c.
6
27. Find the Hamiltonian and Hamilton’s equations of motion for a charge e moving in a
constant uniform magnetic field B described by the following action
!
r
Z t2
2
e
v
αβγ Bα xβ vγ ,
S=
dt −m c2 1 − α2 +
c
2c
t1
where αβγ is the antisymmetric tensor with 123 = 1.
Then, take the direction of the field as the z-axis. Show that the speed of the particle stays
constant throughout the motion. Show that the trajectory is a helix.
28. A photon may be described classically as a particle of zero mass possessing nevertheless a
momentum h/λ = hν/c, and therefore a kinetic energy hν (h is the Planck constant). If the
photon collides with an electron of mass m at rest, it will be scattered at some angle θ with
a new energy hν 0 . Show that the change in energy is related to the scattering angle by the
formula
θ
λ0 − λ = 2λc sin2 ,
2
h
, is known as the Compton wavelength. Show also that the kinetic energy of the
where λc = mc
recoil motion of the electron is
2 λλc sin2 2θ
T = hν
.
1 + 2 λλc sin2 2θ
29. The transverse vibrations of a stretched string can be approximated by a discrete system
consisting of equally spaced mass points located on a weightless string. Show (neglect all
external interactions) that if the spacing is allowed to go to zero, the Lagrangian approaches
the limit
Z
1
L=
dx µ ẏ 2 − T y 02
2
for the continuous string, where T is the fixed tension,
ẏ =
∂y
,
∂t
y0 =
∂y
,
∂x
and x and y are the longitudinal and transversal coordinates of a point of the string, respectively.
What is the equation of motion if the density µ is a function of position?
30. Find the stress-energy tensor of the sine-Gordon field described by the Lagrangian density
1
L = ∂ i φ∂i φ − m2 (1 − cos φ) .
2
What is the trace of the tensor? Evaluate the tensor for a sine-Gordon kink
φ = 4 arctan emγ(x−vt) ,
γ=√
1
.
1 − v2
Find the soliton energy, the momentum carried by the soliton, and the dispersion relation.
What is the mass of the soliton?
7