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An empty balloon weighs W1 on a spring balance. After it is filled with air it weighs W2 on the same balance. Let w be the weight of the air put inside. Ignore the balloon's wall thickness and assume the densities of the air inside and outside are essentially equal. Which statement is correct?
- W2 = W1
- W2 = W1 + w
- W2 < W1 + w
- W2 > W1
Correct answer: W2 = W1
Solution
The enclosed air adds weight w, but the displaced outside air provides an upward buoyant force equal to w (equal densities). These cancel, so the reading is unchanged.
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