Why does an iron nail sink when it is placed on the surface of water?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The gravitational pull of the Earth on the nail is more than the upthrust of water on it
An object in water feels two forces: its weight pulling it down and the upthrust (buoyant force) of the water pushing it up. Iron is denser than water, so the upthrust on the nail, equal to the weight of the small amount of water it displaces, is less than the nail’s weight, and the nail sinks.
Why the other options are incorrect
Option (2) The upthrust of water on the nail is more than the gravitational pull of the Earth on it — Incorrect. A larger upthrust would push the nail up, so it would float, like a cork.
Option (3) The gravitational pull and the upthrust on the nail are equal — Incorrect. Equal forces would leave the nail balanced, floating or staying where it is.
Option (4) Gravity does not act on the nail when it is placed on water — Incorrect. Gravity acts on every object everywhere; it is what pulls the nail down.