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?

(A) The bag of cotton is heavier than the iron bar.
This is because the volume of the cotton bag is much larger than that of the iron bar.
According to Archimedes' principle,an object immersed in a fluid (air) experiences an upward buoyant force equal to the weight of the fluid displaced.
Since the cotton bag occupies more space,it displaces a larger volume of air,resulting in a greater buoyant force acting on it compared to the iron bar.
This buoyant force reduces the apparent weight of the cotton bag on the weighing machine.
Therefore,for the weighing machine to show $100\, kg$ for both,the actual mass of the cotton bag must be greater than that of the iron bar to compensate for the higher buoyant force.