# Trying to understand the tension in a spring with two weights attached [duplicate]

Possible Duplicate:
Why I think tension should be twice the force in a tug of war

For (A) in this image, I would think the tension should be should be 2mg.

However, my book says the weight in (A) should be only mg. I don't understand why the tension should not be the same as in (C) and double the tension in (B).

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## marked as duplicate by Qmechanic♦, Manishearth♦Dec 11 '12 at 10:54

Have you drawn the free-body diagram? –  tpg2114 Nov 27 '12 at 23:31
Possible duplicates: physics.stackexchange.com/q/41291/2451 and links therein. –  Qmechanic Nov 27 '12 at 23:51

In c) glue together the pulley and two weights and you will see that the force on the spring is $2 m g$. If you agree that a) and b) are the same, then you agree that for b) the force is $m g$ only. –  ja72 Nov 29 '12 at 4:33
As I said, a) and b) are in static equilibrium, with equal and opposite forces acting on each side of the spring. Each one is $m g$ from the weights that are hanging. –  ja72 Nov 29 '12 at 20:46