Solution to 2001 Problem 72

Initially the force of the spring on the lower block must be m g upward where m is the mass of the identical blocks. So, initially, the force of the spring on the upper block must also be m g, but now the force points downward. Therefore, when the string breaks, the upper block will feel a force of 2 m g downward, one m g due to the spring and one mg due to gravity. Therefore, the acceleration of the upper block will be \boxed{2g} downward. Therefore, answer (E) is correct.

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