Classical Mechanics - Harmonic Oscillator

Two identical blocks are connected by a spring. The combination is suspended, at rest, from a string attached to the ceiling, as shown in the figure. The string breaks suddenly. Immediately after the string breaks, what is the downward acceleration of the upper block?

A. 0
B. g/2
C. g
D. √2 g
E. 2g

(GR0177 #72)

Solution:

In equilibrium (before the string is cut):
Block 1:

                                    …(Eq.1)

Block 2:
                  … (Eq.2)

After the string is cut:
, and
(minus sign    is because the direction is in downward or parallel with  )

(Eq.1) becomes:

                                                                   ...(Eq.3)

Submit (Eq.2) to (Eq.3):   

Since  

Answer: E