Even though it seems like an anomaly, there is a scientific reason for this phenomenon happening in every day life. It is true that an iron needle sinks in the water while a ship, which is much heavier than an iron needle, floats in the water. To understand the reasons behind this unusual phenomenon, we need to first understand the flotation principle of Archimedes.
What is the principle of Archimedes?
The principle of Archimedes states that on immersion of an object in a liquid, the upward buoyant force experienced by it the same as the amount of displaced fluid, by weight. The salient features of this principle are à
a) Every object fully or partially immersed in the water experiences an upward thrust
b) The volume of the fluid displaced equals the volume of the sunken object
c) The displaced fluid weight is called the buoyancy of the displacing object
d) This principle holds true for all solids and liquids.
Explaining the sinking of an iron needle:
Taking the above given principle into account, it can be said that when an iron needle is immersed in water, ità
a) Displaces some amount of water
b) This displaced water is equal to the weight of the iron needle.
Now since the weight of the iron needle is very less, the water it displaces is also very less. There are also two types of forces working in the opposite direction of the needle here. They are a Buoyant force or upward thrust which tries to force the iron needle to float and
a) Gravitational force or the pull of the earth’s gravity on the needle
In the case of an iron needle because the water displaced is very less, consequently the buoyant force acting in the upward direction is also very less and is unable to overcome the force of gravity or gravitational force acting in the downward direction. This causes the iron needle to sink in the water.
Explaining why an iron ship floats in the water
It is a known fact that objects weigh more on land than in water. To understand the reason behind it we need to know two things a
a) Definition of weight which is nothing but the measure of the pull of gravity on an object
b) Forces acting in opposite directions tend to decrease the intensity of the net force acting on the object.
Thus we can say that in the water, the force of gravity is countered by the buoyant force thereby reducing its impact on the object concerned, thereby making it weigh less in water than on land.
This principle is also applicable on an iron ship thereby making it weigh relatively less in water than on land. But the water displaced, even then, is substantial in amount making it experience a buoyant force of a very high magnitude. This buoyant force or upward thrust is able to overcome the pull of gravity on the ship to a great extent, thus causing it to float in water.
That is why we say that whereas an iron needle sinks in water, an iron ship much heavier than the iron needle floats in it.