we have a copper rod that's completely encased in a steel tube. The steel tube has an inner diameter of 2cm and an outer diameter of 4cm. You mentioned that the stress in the steel tube is 100 N/mm². Now, to find the stress in the copper rod, we need to consider the relationship between the two materials.
Since the copper rod is encased in the steel tube, it experiences a compressive force from the steel tube. The stress in the copper rod will be dependent on the stress in the steel tube. However, without additional information about the material properties and the specific nature of the load, it's not possible to determine the exact stress in the copper rod.