1. A close helical spring has a stiffness of 10 N/mm. Its length when fully compressed, with adjacent coils touching each other, is 400 mm. The modulus of rigidity of the material of the spring is 8 x 10⁴ N/mm². What is the wire diameter and the mean coil diameter if their ratio = 1/10?

2. A close helical spring has a stiffness of 10 N/mm. Its length when fully compressed, with adjacent coils touching each other, is 400 mm. The modulus of rigidity of the material of the spring is 8 x 10⁴ N/mm². If the gap between any two adjacent coils is 2 mm, what maximum load can be applied before the spring becomes solid, i.e., adjacent coils touch?

3. A close helical spring has a stiffness of 10 N/mm. Its length when fully compressed, with adjacent coils touching each other, is 400 mm. The modulus of rigidity of the material of the spring is 8 x 10⁴ N/mm². What is the maximum stress in the spring?