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A glass plate 20 cm long and 0.2 cm thick just makes contact with a water surface in a beaker. Water's surface tension is 72 dyne/cm and the plate weighs 25 g. What additional weight must be placed on the opposite balance pan to keep the balance level?
- 25 g
- 28 g
- 22 g
- 21.3 g
Correct answer: 28 g
Solution
Surface tension acts along the line of contact (perimeter) pulling the plate down. The total downward force is the plate weight plus T times the perimeter. Converting that extra force to grams and adding to 25 g gives about 28 g.
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