An unloaded vertical light spring with elastic constant 
is 80 cm long. A block of mass 1.5 kg is attached to the lower end of the spring with the block resting on the ground while the spring is not deformed (see figure). The upper end of the spring is then slowly raised from a height of 80 cm to 170 cm.
How much work is done during the lifting?
Draw the graph of the force acting on the upper end of the spring against the height.
Draw the graph of the elastic energy of the spring against the height. 
Figure 1: Spring weight.
