## Mechanics 2

### Lagrangian Mechanics

Summary of the concepts you need to be familiar with to solve most GRE problems.

Any set of independent quantities q1, q2, ... , qs, which completely define the position of the system with s degrees of freedom, are called generalized coordinates of the system, and the derivatives are called generalized velocities.

Examples:

• A particle is constraint to move in the x-y plane, the equation of constraint is z = 0, the constraint is holonomic.  Possible generalized coordinates for the system with two degrees of freedom are x, y; r, φ; ... .
• A particle is constraint to move on a circle in the x-y plane, the equations of constraints are z = 0, x2 + y2 - r2 = 0.  The constraints are holonomic.  Possible generalized coordinates for the system with 1 degree of freedom are φ ; φ3, ... .

The Lagrangian of the system is L = T - U.   L is a function of the generalized coordinates and velocities and possibly the time,
L = L (q, v, t), where q = {qi}, v = {vi}, and vi = dqi/dt.

If all forces present are conservative or can do no virtual work, them the equations of motion may be obtained from Lagrange's equations,
d/dt(∂L/∂(dqi/dt)) -  ∂L/∂qi = 0.

The generalized momentum or conjugate momentum or canonical momentum is defined through
pi = ∂L/∂vi.
Note:  If a coordinate qi does not explicitly appear in the Lagrangian, then the coordinate is called cyclic and the corresponding conjugate momentum pi is constant.

The Hamiltonian H of a system is given by  H(q, p, t) = ∑i(dqi/dt)pi - L.
Note:  H(q, p, t) is a function of the generalized coordinates and momenta and possibly the time.

• If the Lagrangian does not explicitly depend on time, then the Hamiltonian does not explicitly depend on time and H is a constant of motion.  [If H does explicitly depend on time, H = H(t), then H is not a constant of motion.]
• If the generalized coordinates do not explicitly depend on time, then H = T + U = E, the total energy of the system.  [If the generalized coordinates do explicitly depend on time, then H is not the total energy of the system.]
• So only if Lagrangian does not explicitly depend on time and the generalized coordinates do not explicitly depend on time, then H = T + U = E and the energy is a constant of motion.

Hamilton's equations of motion:  dqi/dt = ∂H/∂pi,  dpi/dt = -∂H/∂qi.

### Relativity

Summary of the concepts you need to be familiar with to solve most GRE problems.

### Postulates

I. The laws of nature are the same in all inertial reference frames.
II. In vacuum, light propagates with respect to any inertial frame and in all directions with the universal speed c.  This speed is a constant of nature.

Proper time τ:  The time interval between two events in a reference frame where the two events have the same space coordinates.  (They "happen" at the same place.)
In a frame moving with speed v with respect to that frame the time interval between the two events is t = γτ.
γ = (1 - v2/c2),  t > τ, the proper time interval is the shortest time interval.

Proper length L0:  The length (dimension) of an object in a reference frame in which the object is at rest.
In a frame  moving with speed v with respect to that frame the length of the object in the direction of the relative motion is L = L0/γ.  L < L0.  The length of the object in any direction perpendicular to the direction of the relative motion is the same in both frames.

The space-time interval between two events is ds,  ds2 = c2dt2 - |dr|2.   It is a Lorentz invariant quantity, i.e. it is the same in every reference frame.

Relativistic energy and momentum:
E = γmc2p = γmvpc/E = v/c.
E2 = m2c4 + p2c2.
In every reference frame energy and momentum are conserved.

Lorentz transformation:
Consider two reference frames K and K'.  Assume that the coordinate axes in the two frames are parallel and that the origins of the coordinates coincide at t = t' = 0.  Assume that K' is moving with velocity vi with respect to K.  The Lorentz transformation gives the coordinates of a space-time point (x0,x1,x2,x3) = (ct,x,y,z) in K in terms of its coordinates (x'0,x'1,x'2,x'3) = (ct',x',y', z') in K' and vice versa.

.

β = v/c,  β = v/c,  γ = (1 - β2).

A particle moves in K with velocity u = dr/dt.
K' moves with respect to K with velocity v.
The particle's velocity in K', u' = dr'/dt', is given by
u'|| = (u|| - v)/(1 - vu/c2),
u' =  u/(γ(1 - v∙u/c2)).

### Problem 1:

A non-relativistic particle of mass m moves in a plane.  Its position is described by the polar coordinates r and θ.  There exists a potential energy U = kr2, where k is a constant.  Which of the following is the Lagrangian of the particle?

(A)  L =  ½m[(dr/dt)2 + r2(dθ/dt)2] - kr2

(B)  L =  ½m[(dr/dt)2 + θ2] + kr2.

(C)  L =  ½m[θ2(dr/dt)2 + r2(dθ/dt)2] - kr2.

(D)   L =  ½m[(dr/dt)2 + r(dr/dt)(dθ/dt) + r2(dθ/dt)2] - kr2.

(E)  L =  ½m[(dr/dt)2 + 2r(dr/dt)(dθ/dt) + r2(dθ/dt)2] - kr2.

### Problem 2:

A non-relativistic particle of mass m moves in a plane.  Its position is described by the polar coordinates r and θ.  There exists a potential energy U = kr2, where k is a constant.  Which of the following quantities remains constant?

(A)  m[(dr/dt)2 + r2(dθ/dt)2]
(B)  mr2(dθ/dt)2
(C)  kr2
(D)  mr2dθ/dt)
(E)  mr2(dθ/dt)

### Problem 3:

The Lagrangian for a system with generalized coordinate q is L = m(dq/dt)4 -g(q), where g(q) is an arbitrary function of the coordinate.  The canonical momentum conjugate to q is

(A)  m(dq/dt)    (B)  m(dq/dt)3    (C)  g(q)(dq/dt)    (D)  m(dq/dt)2/3    (E)  4m(dq/dt)3

### Problem 4:

Refer to the previous problem.
Which of the following is a constant of motion for this system?

(A)  ½ m(dq/dt)2 + g(q)    (B)  m(dq/dt)2 + g(q)    (C)  m(dq/dt)4 + g(q)
(D)  3 m(dq/dt)4 + g(q)        (E)  8 m2(dq/dt)6 + g(q)

### Problem 5:

In an inertial frame S a particle has momentum (px, py, pz) = (5, 3, √2) MeV/c and a total energy E = 10 MeV.  The speed of the particle as measured in frame S is most nearly

(A)  (3/8) c    (B)  (2/5) c   (C)  ½ c   (D)  (3/5) c    (E)  (4/5) c

### Problem 6:

Which of the following combinations of momentum p' and energy E' could represent the motion of the particle described in the previous problem as observed in another inertial frame S' moving with an unspecified velocity v relative to S.

(A)  p' = (0, 0, 8) MeV/c,        E' = √(128) MeV

(B)  p' = (8, 0, √2) MeV/c,        E' = 10 MeV

(C)  p' = (31, 4, 6) MeV/c,        E' = √(949) MeV

(D)  p' = (50, -30, √(200)) MeV/c,        E' = 100 MeV

(E)  p' = (100, 100, 0) MeV/c,        E' = 10,000 MeV

### Problem 7:

The percentage increase in the energy of a particle whose speed changes from rest to 0.8 c is closest to

(A)  50%   (B)  67%   (C)  75%   (D)  80%    (E)  88%

### Problem 8:

Two spaceships, each measuring 100 m in length in its own rest frame, pass by each other traveling in opposite directions.  Instruments on spaceship 1 determine that the front end of spaceship 1 requires (5/3)*10-7 s to traverse the full length of spaceship 2.  What is the relative speed of the two ships?

(A)  (1/√6)c   (B)  (½)c    (C)  (1/√2)c    (D)  (2/√5)c     (E)  (2/√3)c

### Problem 9:

A proton has kinetic energy 500 GeV.  The momentum of the proton is most nearly

(A)  22 GeV/c   (B)  30 GeV/c    (C)  250 GeV/c    (D)  500 GeV/c     (E)  707 GeV/c

### Problem 10:

A ∑0 particle (ass M1) decays at rest in the laboratory into a Λ0 particle (mass M2) and a massless photon.  The energy of the Λ0 particle is

(A)  M1c2/2   (B)  (M12 + M22)c2/(2M1)   (C)  (M1 + M2)2c2/(2M1)

(D)  (M12 - M22)c2/(2M1)      (E)  (M1 + M2)c2/2