question_answer

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?                       

Answer:

                (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.