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A wire of cross-sectional area 3.0 mm² and natural length 50 cm is fixed at the top, and a 2.1 kg mass hangs from its lower end. Find the elastic potential energy stored in the wire at equilibrium. Young's modulus of the wire material = 1.9 * 10¹¹ N/m², g = 10 m/s².

1. 7.7 * 10⁻⁴ J
2. 1.9 * 10⁻⁴ erg
3. 3.8 * 10⁻⁴ J
4. 1.9 * 10⁻⁴ J

Correct answer: 1.9 * 10⁻⁴ J

Solution

With F = 21 N the extension is about 1.84*10⁻⁵ m, so U = (1/2)*F*extension is approximately 1.9*10⁻⁴ J.

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