(2009? Binzhou simulation) as shown in the figure, in a vertical smooth tube placed inside a light spring, the upper end of the O point and the mouth of the tube A distance of 2x0, a mass of m

A, B, set the ball just moved to the O point when the speed is v, then there is mg?2x0=

1

2

mv2, v=2

gx0

. The ball first accelerates after contacting the spring, so the maximum velocity of the ball is greater than 2

gx0

. Therefore, A is correct and B is wrong.

C, set the spring force and gravity is equal in size, the spring compression is x, then there is mg = kx, k =

mg

x

. And the spring's elastic force is balanced with gravity above point B, then x < x0, then k >

mg

x0

. Therefore, C is wrong.

D, when the ball moves to the lowest point B, the maximum elastic potential energy, according to the law of conservation of mechanical energy, the maximum elastic potential energy of the spring is 3mgx0. Therefore, D is correct.

The choice of AD.