2. A force of 20 lb is required to hold a spring stretched 8 in. beyond its natural length. How much work W is done in stretching it from its natural length to 11 in. beyond its natural length?
3. A spring has a natural length of 28 cm. If a 23-N force is required to keep it stretched to a length of 42 cm, how much work W is required to stretch it from 28 cm to 35 cm?
4. If the work required to stretch a spring 3 ft beyond its natural length is 12 ft-lb, how much work is needed to stretch it 18 in. beyond its natural length?