The length of an elastic string is \(a\) metre when the longitudinal tension is \(4\) N and \(b\) metre when the longitudinal tension is \(5\) N. The length of the string in metre when the longitudinal tension is \(9\) N will be:
| 1. | \(a-b\) | 2. | \(5b-4a\) |
| 3. | \(2b-\frac{1}{4}a\) | 4. | \(4a-3b\) |

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Hooke's law is applicable for:
| 1. | elastic materials only | 2. | plastic materials only |
| 3. | elastomers only | 4. | all of these |

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Two wires of copper having length in the ratio of \(4:1\) and radii ratio of \(1:4\) are stretched by the same force. The ratio of longitudinal strain in the two will be:
| 1. | \(1:16\) | 2. | \(16:1\) |
| 3. | \(1:64\) | 4. | \(64:1\) |

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An elastic string obeying Hooke’s law has a length \(l_1\) metres when the tension is \(4~\text{N},\) and a length \(l_2\) metres when the tension is \(5~\text{N}.\) What is its natural length (i.e., its length when the tension is zero)?
1. \(5l_1-4l_2\)
2. \(5l_2-4l_1\)
3. \(9l_1-8l_2\)
4. \(9l_2-8l_1\)

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