Buoyancy and Archimedes Principle

    If you try to push down a ball filled with air under water, you would feel a great resistance to do it. Let’s say you did, but once you take your hands off, the ball will come over the water and start floating on it. Why is that? Along with that, one may question why a ship floats on water while a small peddle would even sink in water? So, precisely we have an answer to that thanks to buoyancy and especially Archimedes, the famous ancient Greek mathematician.

    Well! Let’s go forward. Buoyancy or upper thrust is the opposite force exerted by a fluid when an object is partially or fully merged in it. This theory can further be described by Archimedes’ principle. Now, think of a beaker full of water to its mouth is put an object in it. After that another empty beaker is placed at its mouth for the displaced water to get it. Archimedes said,

    “when an object is submerged in a fluid, the merged object’s weight is equal to the displaced water’s weight.” Also, we should understand that water gives an opposite force which is equal to the displaced objects weight. That means the buoyant force is equal to the displaced waters weight, which is measured in N(Newtons).

Weight of displaced water = Buoyant force. 

    If the weight of the merged object is less than the weight of the displaced water, the object floats on the water. Ex. A plastic ball floats on water since the buoyant force is higher than its weight. This is called ‘positive buoyancy’.

2.       If the weight of the merged object is higher than the weight of the displaced water, the object floats on the water. Ex. An iron rod sinks in water since the buoyant force is less than its weight. This is called ‘negative buoyancy’.

3.       If the weight of the merged object is equal to the weight of the displaced water, the object floats partially on water. This is called ‘neutral buoyancy’.

As a summary,

if weight of object < weight of water displaced = the object floats

if weight of object = weight of water displaced = the object partially floats

if weight of object > weight of water displaced = the object sinks

So, this is why a pebble sinks in water (weight of the pebble > weight of water displaced by it) and this is why a ship floats (weight of ship < weight of water displaced by the ship). Remember, the ship is also full of air, meaning air is light and has less mass, but if we make the whole ship into an iron rod, then there is no air, resulting the weight of object > weight of displaced water.

To calculate buoyant force in any fluid, we need to find the density of the kind of fluid, volume of the water according to its density and the gravitational acceleration.

buoyancy= N

density = kg/m³

volume = L

gravitational acceleration = m/s² 

Therefore, N = kg/m³  x L x m/s².

Thank you.