You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than the other. Can you say which one is heavier and why?
We know that,
Measured weight of the body = Actual weight of the body - Buoyant force
Therefore,
Actual weight = Measured weight + Buoyant force
As measured weight of cotton and iron is same, actual weight will only depend on the buoyant force.
Buoyant force is given by:
B = Volume of the body × g × Density of air.
Buoyant force is directly proportional to the volume of the body.
As volume of iron is less than the volume of cotton, therefore buoyant force acting on the iron will be less than that of iron.
Hence, actual weight of the cotton will be more than the iron.