(2014?Dongying second model) as shown in the figure, in the fixed inclined smooth rod set a mass of m ring, rod and the horizontal direction of the angle α = 30 °, the circle

A, the ring slides down along the rod, sliding to the bottom of the rod in the process of two forces on the ring to do work, that is, the ring of gravity and the spring tension; so the mechanical energy of the ring is not conserved, if the ring and the spring composed of the system as the object of study, then the mechanical energy of the system is conserved, so the A option is wrong;

B, when the acceleration of the ring along the rod is zero, its speed is the maximum, the kinetic energy is the maximum, at this time the spring In the elongation state, give the ring a diagonal upward tension, so B is wrong;

C, according to the functional relationship can be seen, when the ring slides to the very bottom of its speed is zero, the gravitational potential energy is converted into elastic potential energy, at this time the maximum elastic potential energy is equal to the reduction of gravitational potential energy that is mgh, so C is correct;

D, the spring turned 60 ° angle, this time, the spring is still original length, and the ring as the object of study, the spring is the original length, the spring is still the original length, the spring is still the original length. td>2 =

1

1

2

mv2, so D is correct.

Therefore: CD.