While testing an elevator in a multistorey building, a physics student uses a spring balance whose stiffness constant is not yet known. She first suspends a 2.00kg standard mass from the balance at rest and notes that the pointer shifts downward by 4.0cm. Later, riding in the elevator that is accelerating uniformly upward at 1.5ms−2, she hangs an unknown body from the same balance and observes a downward shift of 7.0cm. Assuming the spring obeys Hooke’s law and its own mass is negligible, determine the mass of the unknown body. Show all necessary steps in your reasoning.