The modulus of normal elasticity (Young's modulus) is greater for steel than for copper. Therefore if the springs are of equal dimensions and are to be stretched by the same amount, a
132 SOLUTIONS
greater force is necessary for the steel spring than for the copper one. So the first spring requires that more work should be done.
79. (See previous problem.) If the process of stretching is carried out with equal forces for both springs, the steel spring will be stretched less than the copper one. Therefore more work will be done this time on stretching the copper spring.