One end of a light spring of spring constant k is fixed to a wall and the other end is tied to a block placed on a smooth horizontal surface. In a displacement, the work  done by the spring is 1/2 kx2 . The possible cases are
(a) the spring was initially compressed by a distance x and was finally in its natural length
(b) it was initially streched by a distance x and finally was in its natural length
(c) it was initially in its natural length and finally in a compressed position

(d) it was initially in its natural length   and finally in a stretched position.

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1 Answers

(a) the spring was initially compressed by a distance! and was finally in its natural length 

(b) it was initially streched by a distance x and was in its natural length 

Explanation: 

Since in the conditions of (c) and (d) spring does not exert a force that is in the direction of displacement but in the opposite, so the work done will be negative. 

Only in (a) and (b) the work done will be positive.

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