A rock weighs 2.25 Newtons when suspended in air. In water, it appears to weigh only 1.8 Newtons. Why?
There is a buoyant force acting vertically upwards on the rock.
The most likely scenario was that it was hung on a Newton meter or similar whilst being fully/partially submerged in water.
But let's first consider the method of determining the weight of the rock in air with a Newton meter. There would be two vertical forces acting on the rock: tension,
In the case of full/partial submersion there would be three vertical forces acting on the rock: tension,
If you would like to know how to calculate the buoyant force see the answer to this question: https://socratic.org/questions/how-do-you-calculate-the-buoyant-force-of-an-object