Conceptual Questions Based on Class 11 Physics Chapter 9 Mechanical Properties of Solids

Conceptual Questions Based on Class 11 Physics Chapter 9 Mechanical Properties of Solids

Q.1. In stretching a wire, we have to perform work. Why?

Solution. When a wire is stretched, interatomic forces of attraction come into play. In order to stretch the wire, work has to be done against these forces.

Q.2. The ratio stress/strain remains constant for a small deformation. What happens to this ratio if deformation is made very large?

Solution. When the deforming force exceeds the elastic limit, the strain increases more rapidly than stress. Hence the ratio of stress/strain decreases.

Q.3. Two identical springs of steel and copper are equally stretched. On which more work will have to be done?

Solution. Young’s modulus of steel is greater than that of copper. In order to produce same extension, large force will have to be applied on the steel spring than that on the copper spring. Hence more work will be done on the steel spring.

Q.4. If two identical springs of steel and copper are pulled by applying equal forces, then in which case more work will have to be done?

Solution. Steel spring will be stretched to a lesser extent. Now more work will be done on the copper spring.

Q.5. What happens to the work done in stretching a wire?

Solution. The work done in stretching a wire is stored in it as elastic potential energy.

Q.6. Why does a wire get heated when it is bent back and forth?

Solution. When a wire is bent back and forth, its deformations are beyond elastic limit. The work done against interatomic forces is no longer stored totally in the form of potential energy. The crystalline structure of the wire gets affected and work done is converted into heat energy.

Q.7. A hard wire is broken by bending it repeatedly in alternating directions. Why?

Solution. When the wire is subjected to repeated alternating strains, the strength of its material decreases and the wire breaks.

Q.8. Following are the graphs of elastic materials. Which one corresponds to that of brittle material? 

         

Solution. Graph (a) represents a brittle material as it indicates a very small plastic range of extension.

Q.9. Two different types of rubber are found to have the stress-strain curves as shown in Fig.

(a)   In which significant ways do these curves differ from the stress-strain curve of a metal wire shown in Fig.?

(b)   A heavy machine is to be installed in a factory. To absorb vibrations of the machine, a block of rubber is placed between the machinery and the floor. Which of the two rubbers A and B would you prefer to use for this purpose? Why?

(c)   Which of the two rubber materials would you choose for a car tyre? 

Solution. (a) The stress-strain curves for rubber differ from the stress-strain curve for a metal in following respects:

(i) Hooke’s law is not obeyed even for small stresses.

(ii) There is no permanent set (residual strain) even for large stresses.

(iii) There is large elastic region for both types of rubber.

(iv) Neither material retraces the curve during unloading. Thus both materials exhibit elastic hysteresis.

(b)   The area of the hysteresis loop is proportional to the energy dissipated by the material as heat when the material undergoes loading and unloading. A material for which the hysteresis loop has larger area would absorb more energy when, subjected to vibrations. Therefore, to absorb vibrations, we would prefer rubber B.

(c)   In car tyre, the energy dissipation must be minimised to avoid excessive heating of the car tyre.

As rubber A has smaller hysteresis loop area (and hence smaller energy loss), so it is preferred to B for a car tyre.

Q.10. Elasticity has a different meaning in physics than that in daily life. Comment.

Solution. In daily life, a body is said to be elastic if a large deformation or strain is produced on applying a given stress on it. In physics, elasticity is the property of the material of a body by virtue of which it opposes any change in its size or shape when a stress is applied on it. Thus, a body will be more elastic if a small strain is produced on applying a given stress on it.

Conceptual Questions Based on Class 11 Physics

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