245. Describe how to determine whether an equilibrium is stable or unstable when (d²U/dx®), = 0. %3D

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PROBLEMS
97
245. Describe how to determine whether an equilibrium is stable or unstable when
(d²U/dx²), = 0.
246. Write the criteria for determining whether an equilibrium is stable or unstable
when all derivatives up through order n, (d"U/dx"), = 0.
247. Consider a particle moving in the region x>0 under the influence of the potential
U(x) = Up
where U, = 1 J and a = 2 m. Plot the potential, find the equilibrium points, and
determine whether they are maxima or minima.
248. Two gravitationally bound stars with equal masses m, separated by a distance d, re-
volve about their center of mass in circular orbits. Show that the period 7 is propor-
tional to d/2 (Kepler's Third Law) and find the proportionality constant.
2-49. Two gravitationally bound stars with unequal masses m, and mg, separated by a dis-
tance d, revolve about their center of mass in circular orbits. Show that the period 7
is proportional to d³/² (Kepler's Third Law) and find the proportionality constant.
2-50. According to special relativity, a particle of rest mass mo accelerated in one dimen-
sion by a force F obeys the equation of motion dp/dt = F Here p = mov/(1 –
v/c*) !/2 is the relativistic momentum, which reduces to mov for v²/c² <1. (a) For
the case of constant F and initial conditions x(0) = 0 = v(0), find x(t) and v(t).
(b) Sketch your result for v(t). (c) Suppose that F/mo = 10 m/s² ( = g on Earth).
How much time is required for the particle to reach half the speed of light and of
99% the speed of light?
2-51. Let us make the (unrealistic) assumption that a boat of mass m gliding with initial
velocity vo in water is slowed by a viscous retarding force of magnitude bu², where b
is a constant. (a) Find and sketch v(t). How long does it take the boat to reach a
speed of w/1000? (b) Find x(t). How far does the boat travel in this time? Let m =
200 kg, y = 2 m/s, and b = 0.2 Nm-2s2.
2-52. A particle of mass m moving in one dimension has potential energy U(x) =
U,[2(x/a) 2 - (x/a) *], where U, and a are positive constants. (a) Find the force
F(x), which acts on the particle. (b) Sketch U(x). Find the positions of stable and
unstable equilibrium. (c) What is the angular frequency w of oscillations about the
point of stable equilibrium? (d) What is the minimum speed the particle must have
at the origin to escape to infinity? (e) At t = 0 the particle is at the origin and its ve-
locity is positive and equal in magnitude to the escape speed of part (d). Find x(4)
and sketch the result.
2-53. Which of the following forces are conservative? If conservative, find the potential
energy U(r). (a) F, = ayz + bx + c, F, = axz + bz, F, = axy + by. (b) F, =
- ze*, F, = Inz, F, = e=* + y/z. (c) F = e,a/r(a, b, c are constants).
2-54. A potato of mass 0.5 kg moves under Earth's gravity with an air resistive force of
- kmv. (a) Find the terminal velocity if the potato is released from rest and k =
0.01 s-. (b) Find the maximum height of the potato if it has the same value of k,
98
2/ NEWTONIAN MECHANICS–SINGLE PARTICLE
Transcribed Image Text:ll Jazz LTE 2:48 PM @ 76% ( Classical-Dynamics-of-Particles-and-... PROBLEMS 97 245. Describe how to determine whether an equilibrium is stable or unstable when (d²U/dx²), = 0. 246. Write the criteria for determining whether an equilibrium is stable or unstable when all derivatives up through order n, (d"U/dx"), = 0. 247. Consider a particle moving in the region x>0 under the influence of the potential U(x) = Up where U, = 1 J and a = 2 m. Plot the potential, find the equilibrium points, and determine whether they are maxima or minima. 248. Two gravitationally bound stars with equal masses m, separated by a distance d, re- volve about their center of mass in circular orbits. Show that the period 7 is propor- tional to d/2 (Kepler's Third Law) and find the proportionality constant. 2-49. Two gravitationally bound stars with unequal masses m, and mg, separated by a dis- tance d, revolve about their center of mass in circular orbits. Show that the period 7 is proportional to d³/² (Kepler's Third Law) and find the proportionality constant. 2-50. According to special relativity, a particle of rest mass mo accelerated in one dimen- sion by a force F obeys the equation of motion dp/dt = F Here p = mov/(1 – v/c*) !/2 is the relativistic momentum, which reduces to mov for v²/c² <1. (a) For the case of constant F and initial conditions x(0) = 0 = v(0), find x(t) and v(t). (b) Sketch your result for v(t). (c) Suppose that F/mo = 10 m/s² ( = g on Earth). How much time is required for the particle to reach half the speed of light and of 99% the speed of light? 2-51. Let us make the (unrealistic) assumption that a boat of mass m gliding with initial velocity vo in water is slowed by a viscous retarding force of magnitude bu², where b is a constant. (a) Find and sketch v(t). How long does it take the boat to reach a speed of w/1000? (b) Find x(t). How far does the boat travel in this time? Let m = 200 kg, y = 2 m/s, and b = 0.2 Nm-2s2. 2-52. A particle of mass m moving in one dimension has potential energy U(x) = U,[2(x/a) 2 - (x/a) *], where U, and a are positive constants. (a) Find the force F(x), which acts on the particle. (b) Sketch U(x). Find the positions of stable and unstable equilibrium. (c) What is the angular frequency w of oscillations about the point of stable equilibrium? (d) What is the minimum speed the particle must have at the origin to escape to infinity? (e) At t = 0 the particle is at the origin and its ve- locity is positive and equal in magnitude to the escape speed of part (d). Find x(4) and sketch the result. 2-53. Which of the following forces are conservative? If conservative, find the potential energy U(r). (a) F, = ayz + bx + c, F, = axz + bz, F, = axy + by. (b) F, = - ze*, F, = Inz, F, = e=* + y/z. (c) F = e,a/r(a, b, c are constants). 2-54. A potato of mass 0.5 kg moves under Earth's gravity with an air resistive force of - kmv. (a) Find the terminal velocity if the potato is released from rest and k = 0.01 s-. (b) Find the maximum height of the potato if it has the same value of k, 98 2/ NEWTONIAN MECHANICS–SINGLE PARTICLE
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