Question

you have a bag of cotton and an iron bar each indicating a mass of 100kg when measured on a weighing machine in reality one is heavier than the other can you say which one is heavier and why​

Answer 1

We know that true weight = apparent weight + upthrust. The cotton bag is heavier than the iron bar. This is due to the reason, that the bag of cotton which has more volume (as it has less density) than the iron bar (which has more density), experiences more upthrust due to air.

Concept Note - Same mass does not necessarily means same buoyant force.

Hope this will help you ☺

Answer 2

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The bag of cotton is heavier than the bar of iron. The cotton bag has a larger air thrust than the iron bar. The weighing machine therefore indicates a smaller cotton bag weight than its actual weight.