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 -$\frac{1}{4}\mathrm{a}$                                       

4. 4a - 3b

Hooke's law is applicable for:

1. Elastic materials only

2. Plastic materials only

3. Elastomers only

4. All of these

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

The length of elastic string, obeying Hooke's law is $l_1$ metres when the tension is 4N, and $l_2$ metres when the tension is 5N. The length in metres when the tension is 0N will be-

1. $5l_1-4l_2$                                

2. $5l_2-4l_1$

3. $9l_1-8l_2$

4. $9l_2-8l_1$