We all know that things float on the surface of water or any other liquid, but have you ever thought why? Why when we swim in a swimming pool, our body feels lighter than usual? Have you ever wondered why a ship made of iron and steel metals floats on the surface of water whereas a small iron nail sink in the water? Well, all these questions can be answered by taking buoyancy and buoyant force in consideration. Let us understand what is buoyancy by taking some common examples in the given article below.
When we place an empty plastic bottle on the surface of water, it floats. We all know that gravitational force act on everything and pulls it downwards. So what is it that keeps the plastic bottle afloat on the surface of water? The water exerts an anti gravitational force on the plastic bottle and hence causing it to float. This phenomenon which causes to float on the surface of water is known as buoyancy.
Buoyant force is defined as the upward force exerted by water or any other liquid opposite to the gravity which makes an object to float on the surface of it.
In the above example, the plastic bottle floats as the buoyant force exerted by the water is greater than its weight which is nothing but the gravitational force exerted by the earth. If we want the plastic bottle to be completely immersed into the water, then we need to apply an external downward force which is equal to the gravitational force in order to balance it.
All the objects experience buoyant force when they are immersed fully or partially in a fluid. The magnitude of the buoyant force depends upon the density of the fluid in which the object is immersed.
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The buoyant force is exerted in the upward direction, opposite to the gravitational force hence it is also known as Upthrust force.
As buoyant force is also a type of force, the SI unit of it is Newton (N)
The formula for calculating buoyant force is
where, Fbuoyant 🡪 Buoyant force
🡪 Density of the fluid
g 🡪 Gravitational acceleration
V 🡪 Volume of the object
Whenever we talk about buoyant force or buoyancy, this question always arises that why some objects float on the surface of water while se objects sink? When water exerts force on every object placed on the surface of it then why some objects float and some does not? Let's resolve this query by taking a common example.
If we place an iron nail and a cork on the surface of water and both of equal masses, we observe that piece of cork floats on the surface of water whereas the iron nail sinks even the masses of both of them are same.
Iron nail and the cork placed on the surface of water
Picture source : NCERT
This is because their densities are different. The density of the cork is less than the density of the water which means, the buoyant force exerted by the water on the cork is greater then the weight of the cork which is nothing but gravity hence it floats. Whereas in the case of iron nail, the density of the iron nail is greater than the density of water which means that the buoyant force exerted by the water on the nail is less then its weight hence it sinks.
Therefore, the object placed on the surface of water will float or sinks will completely depend on the density of the object as compared to the density of the water.
Archimedes’ Principle is a fundamental law of buoyancy of fluids discovered by a Greek scientist named Archimedes and it is stated as follows:
If we fully or partially immersed a body in a fluid, the upward force experienced by the body is equal to the weight of the fluid displaced by it when it is immersed in the fluid.
This principle explains that when a body is immersed fully or partially in the water or in any other liquid, the amount of buoyant force exerted by the liquid and experienced by the object is equal to the fluid displaced by it in weight. Archimedes principle has many applications from designing ships and submarines to determining the density of a liquid.
In order to calculate buoyancy more accurately and also to determine the purity of a substance, the concept of relative density is necessary to understand.
Relative density of any substance is defined as the ratio of density of a substance to the density of water.
Relative density=Density of a substanceDensity of water
Since, It is a ratio of same quantities, relative density has no unit.
There are several applications of buoyancy. Some of the common natural and human made applications of buoyancy are stated below :
Ship 🡪 Did you ever thought why ship float on water even after having such a high weight? This is because the density of ship less as compare to water due to a hollow-like structure. Hence, according to Archimedes principle the buoyant force exerted by water on it is quite high to support the ship and make it float.
Submarine 🡪 Submarines are provided with a large ballast tank which gets filled up when the submarine submerge in the water which increases its weight then the buoyant force and when the submarine needs to get out of the water, the tank releases its water to make it lighter then the buoyant force.
Fishes 🡪 Fishes have air sacks which they fill in order to increase their weight and move deep inside the water and when they need to move at the surface of water, they empty the air sacks which decreases their weight and the buoyant force automatically move them upward.