Final answer:
Special relativity and general relativity are used in engineering under different circumstances. Special relativity deals with objects moving at speeds close to the speed of light, while general relativity is used in situations involving gravity.
Step-by-step explanation:
Special relativity and general relativity are used in engineering under different circumstances. Special relativity is used when dealing with objects moving at speeds close to the speed of light, such as particles in accelerators. General relativity, on the other hand, is used when gravity is involved, such as in the design of satellites or in calculating the bending of light around massive objects like black holes.