Question

An iron nail sinks in water but a ship made from iron floats in water. Why?
A. The weight of the nail is more than the upthrust experienced by it.
B. The average density of the ship is less than the density of water.
C. Both A and B
D. Unsure

Answer 1

Hint: We know that when a body’s density is less than the density of water, the body floats in water. If the density of the body is greater than the density of water, then the body sinks in water. We consider the case of ships and iron nails and discuss why ships have a lower density than water.

Complete step-by-step solution:
In the question, it is said that the ship made of iron floats in water while an iron nail sinks.
We know that the floating and sinking of an object in water is determined by the amount of water it displaces i.e. the buoyant force applied on the object.
We can say that, for an object to float
Force, $\text{F=}{{\text{F}}_{\text{B}}}$; where ${{\text{F}}_{\text{B}}}$ is the buoyant force which is the weight of the water displaced by the object.
Let us consider the ship.
Even though the ship is large; larger than the nail, the inside part of the ship is hollow. This hollow space of the ship is filled by air; hence it reduces the average density of the ship to lower than that of water.
Therefore, since the density of the ship is less than that of water, it floats in the water.

Hence case B is correct.

Now, let us consider the iron nail.
We know that the density of iron is greater than the density of water. Therefore the weight of the nail is more than the buoyant force or the upthrust experienced by the nail. Hence the iron nail sinks in water.
Therefore case A is also correct.
Hence the correct answer is option C.

Note: According to the Archimedes principle, we know that when an object is immersed in liquid the weight of the liquid displaced will be equal to the weight of the object.
In this case, the ship displaces a sufficient amount of water equal to the weight of the ship and thus attains a floating equilibrium. In case of the iron nail, the weight of the needle is more than the weight of the water displaced. Hence nail sinks and ship floats.
When an object is partly or fully immersed in a liquid, the liquid exerts a force on the object. This force exerted is known as buoyant force or buoyancy.