Question

A cork, a stone and an iron block of equal volume are fully immersed in water. Will they suffer the same

loss in weight? Explain.​

Answer 1

Answer:

As the two objects undergo the same loss of weight when submerged in water then they must displace the same volume of water. Consequently when the objects are in air then they will also displace the same amount of air, and as a result they will also lose the same amount of weight as each other in air. This weight loss in air is a lot less than in water as air is much less dense than water and therefore the weight displaced is a lot less, but nevertheless our buoyancy in air reduces our body weight (for example) by around 0.12% compared to if there was no atmosphere.

But the question doesn’t talk about the weight loss in air, rather just the “weight of these objects” in air. Clearly the weight of the objects themselves is different from the weight loss due to buoyancy. We know that the objects displace the same volume, but they may well have different (average) densities and therefore have different weights themselves.

So the answer to the question as worded is that no, the weight of the objects in air may differ as they may have different densities. But then the weights of the objects may also differ when in water for the same reason so it’s a strange comparison to make. You have to wonder whether the question meant to ask about the loss of weight of objects in air, or perhaps the author mistakenly imagined that buoyancy didn’t exist in air and so the “in air” statement really meant to imply being in a non-buoyant environment.