(2014?Weihai Model) As shown in the figure, the mass of two objects A and B are both m, connected by a light spring, and B is placed on the horizontal ground. With a vertical downward size

A, force F when A is in equilibrium, that is, gravity, elastic force, F, the combined force is zero, after the withdrawal of force F spring elastic force and A gravity is unchanged, then the combined force on A and F is the same as the inverse, that is, the direction of the combined force vertically upward, then the direction of the acceleration vertically upward, the object is in the state of super-heavy, A is correct;

B, before the withdrawal of force F to AB as a whole as a research object, according to the Equilibrium conditions: N = F + 2mg, the moment of withdrawal of force F, the spring elasticity is too late to change, that is, the force on the B have not changed, by the ground support is still equal to F + 2mg, according to Newton's third law, the size of the pressure on the ground B for the F + 2mg, B is wrong;

C, the moment of withdrawal of the force F, the B object of the combined force is still zero, zero acceleration, C is wrong;

D, the moment of withdrawal of force F, the B object of the combined force is still zero, acceleration is zero, C is wrong;

D, after the withdrawal of force F, the system only gravity and spring elasticity work, so A, the spring and the earth composed of the system of mechanical energy conservation, D is correct;

The choice: AD.