Final answer:
The spring balance will show the weight of the submerged aluminum minus the buoyant force. Since the buoyant force equals the weight of the displaced water, the balance will register less than the aluminum's weight in air.
Step-by-step explanation:
The reading on a spring balance when a piece of aluminum is submerged in water will be less than its weight in the air due to the buoyant force acting on it. This buoyant force is equal to the weight of the fluid displaced by the submerged object, according to Archimedes' principle. The balance measures the net force on the object, which is the weight of the aluminum minus the buoyant force. The balance will register a lower reading compared to when the aluminum is weighed in air. For example, if you have an aluminum piece that weighs 1 N in air, and when submerged it displaces water that would otherwise weigh 0.3 N, the spring balance would register 1 N - 0.3 N = 0.7 N. This new measurement is the apparent weight of the submerged aluminum. The scenario is analogous to weighing a subject in a hydrostatic weighing device, which is used for body density determination as mentioned in the reference material.