a) State Hooke's Law and write its mathematical expression, defining all symbols used. (3 marks) b) A spring of natural length 20 cm is suspended vertically from a fixed support. When a mass of 200 g is attached to its lower end, the spring stretches to a length of 25 cm. Calculate: i) the spring constant of the spring. (3 marks) ii) the elastic potential energy stored in the spring at this extension. (3 marks) iii) the length of the spring when a mass of 350 g is attached to it, assuming the elastic limit is not exceeded. (3 marks) c) A machine has a velocity ratio of 6 and an efficiency of 75%. Calculate the effort required to lift a load of 900 N using this machine. (3 marks) [Take g = 10 m/s²]