A wagon weighing 2,000 kg and moving at 0.69 m/s has to be brought to rest by a buffer. Compute the number of springs that would be required in the buffer stop to absorb the energy of motion during a compression of 15 cm. Each spring has 15 coils, made of 2 cm wire, the mean diameter of the coils being 20 cm and G=0.8 x 10' N/mm². Also, determine the stiffness of spring.