Nantong photography ceiling

In the process of A, B and A moving from the lowest point to the highest point, the elastic force of A, B and the spring does positive work, so the mechanical energy is not conserved and the total mechanical energy increases, so A is wrong and B is right.

C. at the lowest point, a was originally balanced. At the moment when the thin thread is cut, the elasticity of the spring remains unchanged. If the resultant force f on a is equal to mg and the direction is vertical upward, according to Newton's second law, the acceleration is a=F and M=mgM, and the direction is vertical upward. According to the symmetry of simple harmonic vibration, the acceleration when a moves to the highest point is a=mgM, and the direction is vertical downward, and the acceleration is not zero.

When D and A are at the lowest point, the elongation of the spring is x 1=(M+m)gk, and when A moves to the highest point, the elongation of the spring is x2=mgk, so the rising height of A is h=x 1-x2=Mgk.

For A, according to the functional relationship, the work done by the spring to A is equal to the increase of the gravitational potential energy of A, so the work done by the spring to A is W = MGH = M2G2k. Therefore, d is correct.

So: BD.