What is the acceleration of the upper block after the string breaks?

[eprparadox]

Homework Statement

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 above (THE FIGURE IS ATTACHED). 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) Sqrt(2)*g
(E) 2g

Homework Equations

F = -kx
F = mg

The Attempt at a Solution

I'm having trouble labeling accurately the forces on the top block.

For the bottom block, when the system is at rest, there should be a downward force of F = mg and an upward force of F = kx, leaving mg - kx = 0.

Now for the top block, initially, there is an upward tension force, and a downward force of F=mg. How do I describe the force exerted by the the spring + lower block on the upper block? Could I just treat it like the spring isn't there and treat it like a mass of 2m (m for the upper block + m for the lower block) and say 2mg - T = 0. And then when the string is cut, the tension goes away and we get 2mg = m*a, so that a = 2g?

Thanks a lot ahead of time.

 

[collinsmark]

Now for the top block, initially, there is an upward tension force, and a downward force of F=mg. How do I describe the force exerted by the the spring + lower block on the upper block? Could I just treat it like the spring isn't there and treat it like a mass of 2m (m for the upper block + m for the lower block) and say 2mg - T = 0. And then when the string is cut, the tension goes away and we get 2mg = m*a, so that a = 2g?

Sounds reasonable to me.

You could present a little more convincing argument by treating the spring as a spring immediately after the string is cut. Thus there are two forces acting on the top block (taking the top block in isolation): gravity (acting on the top block) and the force from the spring. Either way you'll get the same answer though.

 

[eprparadox]

Thank you very much.