Introduction
When an object is completely immersed in a liquid, it experiences a buoyant force that is equal to the weight of the liquid displaced by the object. This is the Archimedes principle.
The buoyant force is exerted on an object = Weight of fluid that the object displaces Â
Pressure in Fluids
- Easily flowing substances are called fluids. All liquids and gases are fluids.
- Fluids also have weight so fluid also exerts pressure on the container they are placed in.
- A fluid exerts pressure in all directions, including upwards.
- As per Pascal’s law, the pressure exerted in any confined mass of fluid is transmitted uniformly in all directions.
Buoyancy
Archimedes’ Principle shows that when an object is placed in a liquid, the liquid exerts an ‘upward force’ on the object.
When an object is partially or completely immersed in a fluid, an upward force acts on it. This force is called an upthrust or buoyant force.
The upward force acting on an object immersed in a liquid is called ‘Buoyant Force’.
It is due to the buoyant force that a heavy object seems lighter in water.
There is an increase in the pressure exerted by the liquid as we go deep. As we lower an object into a liquid increasing the depth, the greater will be the upward force on the object.
The magnitude of the buoyant force
The volume or magnitude of the buoyant force being exerted on an object depends on:
The object’s volume – When an object is submerged in a liquid, the liquid exerts an upward force on it. This upward force is determined by how much mass the object has and how deep the object is submerged. And when the object is completely immersed, the buoyant force is maximum and remains constant.
Note- ‘Buoyant force’ does not depend upon the nature of the material any two bodies having equal volume will experience an equal upward force.
The liquid’s density – A liquid with high density exerts greater upward force as compared to a liquid with lesser density.
Sea water exerts more upward force than fresh water, so it is easier to swim in seawater.
Iron blocks float on mercury because it exerts a very high upward force.
Meaning of ‘Displaced Liquid’
When an object is immersed in a bucket full of water, it displaces some of the water which overflows from the bucket. When the object is completely immersed in water, then the volume of water displaced will be equal to the volume of the object itself.
Applications of Archimedes’ Principle
·      In designing ships and submarines
·      To check the purity of milk using a lactometer
·      To check the density of liquids using a hydrometer
·      In determining the relative density of a substance
Will an object Sink or Float?
As shown by Archimedes’ Principle, the weight of an object acts downward and the buoyant force acts upward. Now,
- If Weight > Buoyant force → object will Sink
- If Weight = Buoyant force → object will float in the liquid
- If Weight < Buoyant force → object will rise in the liquid and then float
Let us assume that W is the weight of a body and FB is the buoyant force acting on it.
If W > FB, then the body sinks → If the object is denser than the liquid, then it sinks into the liquid.
If W < FB, then the body floats → An object with a density less than the liquid floats on the liquid.
An object will float on a liquid if the weight of the liquid displaced by it is equal to its weight.
An object will float in a liquid if the buoyant force that pushes it upwards is greater than the downward force of its weight.
The density of floating objects
An object will float on liquid if its density is less than that of the liquid, The density of cork, wood, and ice is less than water so they will float on water.
An object will float on liquid if its density is equal to that of the liquid, Density of tar is almost equal to water so it will float and remain submerged in water.
An object will sink in a liquid if its density is more than that of the liquid, Iron, Glass, and gold have a higher density than water, so they sink in water.
Why do ships Float?
Ships make use of Archimedes’ Principle. A ship made of iron and steel floats on water because its average density is less than that of water, due to the presence of a lot of air space in it.
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The upward force acting on an object immersed in a liquid is called ‘Buoyant Force’. When an object is wholly immersed in a liquid, it displaces liquid that is equal to its weight due to a buoyant force that it experiences. While stepping into a bathtub, while watching a ship float, and more such examples. Many factors make a ship stay afloat on water. First is their distinct shape, the second is that they are mostly hollow which reduces their density, and finally the water they displace is more than their weight making the buoyant force help them stay afloat. An object will float in a liquid if the upward buoyant force exerted on the object is more than the downward force exerted by the object's weight. Archimedes Principle FAQs
What is buoyant force?
What is the Archimedes Principle?
Where can you see Archimedes' principle in action?
Why don't ships made of metal sink in water?
When will an object float in a liquid?
