Question Answers

Elasticity

1. Steel bridges are declared unsafe after a few decades of use. Why?

When bridges are in use, they are subjected repeatedly to the action of alternating deforming forces through decades of time. Its elastic behavior goes on decreasing. This is called elastic fatigue. After a long use, it may give large value of strain for a small stress and may also lead to the collapse of the bridges. Hence, the bridges are declared unsafe after a long use.

2. Compare the mechanical properties of a steel cable, made by twisting many thin wires together, with those of solid steel rod of the same diameter.

When a steel cable is made by twisting many thin wires, the length is larger than In case of solid steel rod. Thus, more stress can be applied on the twisted cable giving more extension without breakage. Thus, the mechanical strength of the twisted cable is better than that of the solid steel rod.

3. Why are rubbers used as vibration absorbers?

Rubbers are used as vibration absorbers because they have large elastic hysteresis loops. If a rubber is placed between the vibrating body and the floor, the rubber is compressed and released in each cycle of vibration. The workdone by deforming force is greater than the workdone by the rubber in returning to its original shape. In the process of deformation and restoration, a particular amount of energy is absorbed. During vibration, it can store vibration kinetic energy and releases slowly. Hence rubbers are used as vibration absorbers.