课程名称︰力学 课程性质︰系必修 课程教师︰高英哲 开课学院：理学院 开课系所︰物理学系 考试日期（年月日）︰2009.1.14 考试时限（分钟）：180 是否需发放奖励金：是 试题 : Total score: 110pts Do all the problems. Please put down the details of your calculations/ derivation as well as your reasoning. Good luck !! 1.Please answer true (○) or false (╳) to the following questions (10pts): (a) The Lagrangian formulation of the mechanical system is a new theory for classical mechanics beyond Newtonian mechanics. (2pts) (b) If a system is invariant with respect translation in the x-direction, the linear momentum px is conserved. (2pts) (c) All orbits in a central field are closed. (2pts) (d) H = E implies energy conservation. (2pts) (e) If a force Ｆ is conservative, ▽×Ｆ = 0. (2pts) 2.(30pts) The point of suspension of a simple pendulum of length l and mass m is attached to a support which is driven horizontally with time. (a) Find the Lagrangian of the system in terms of generalized coordinates (y,θ), and equation of motion for θ. (10pts) (b) For a sinusoidal motion of the support with y = y0cosωt, find pθ. Derive the Hamiltonian. (10pts) (c) Find the steady-state solution to the equation of motion for small angular displacements. Under which condition is the motion unstable? (10pts) 3.(30pts) A particle of mass m is constrained to move on the inside surface of a smooth cone of half-angle α. The particle is subject to a gravitational force. (a) Find the Lagragian and equations of motion of the system in generalized coordinates θ and r. (10pts) (b) Find the total magnitude of the force of constraint for a circular orbit by rewriting the Lagrangian in θ, r and z, and using the Lagrange multipliers and constraints. (10pts) (c) Show that a circular orbit is stable by finding the equation of the motion of a small perturbation in the radial direction. Find the oscillation frequency of the small perturbation. (10pts) 4.(10pts) A particle of mass m moves in an attractive spherical potential U(r) = -V, r < R; U(r) = 0, r > R. (a) Plot the effective potential for a given angular momentum L. (5pts) (b) Discuss possible trajectories for different values of L and energy E. (5pts) 5.(10pts) A satellite orbits around the Earth in a circular orbit at radius R and velocity v. A rocket fires in the radial direction, giving the satellite an outward radial velocity v in addition to its original velocity. Ignore the difference in the satellite mass before and after the rocket burn. (a) Find the ratio of the new energy and angular momentum to the old. (5pts) (b) What is the new orbit of the satellite? (5pts) 6.(20pts) A particle of mass m moves under a conservative force with potential energy V(x) = cx/(x^2+a^2), where c is a constant. (a) Find the position of stable equilibrium, and the period of small oscillations about it. (10pts) (b) If the particle starts from this point with velocity v, find the range of v for which it (1) oscillates, (2) escapes to +∞. (10pts) Useful Formulae ‧Orbit Equation: d^2 u μ ─── + u = - ─── F (1/u), u = 1/r dθ^2 (ul)^2 ‧Keplerian Orbits: 2 2El α/r = 1 + εcosθ; α = l^2/μk; ε = √(1+───) μk^2 --

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