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Thrust and Pressure

Thrust and pressure

Earth is surrounded by a lot of gases. This envelope of gases around our planet is called atmosphere. Atmosphere is vital for the survival of all life forms on our planet. As gases have mass, hence, they exert pressure on their surroundings. Atmosphere consists of gases. Hence, it also exerts pressure on the earth’s surface along with all the life forms living on it. This pressure exerted by the atmosphere is called atmospheric pressure.

We can now define atmospheric pressure as:

The force exerted on a unit area by a column of air above the earth’s surface is called atmospheric pressure.

The value of atmospheric pressure in the SI system is 100000 N/m2 or 100000 Pa.

Variation in Atmospheric Pressure

Atmospheric pressure varies with:

  • Height — With increase in height from the sea level, atmospheric pressure decreases. Following two factors are mainly responsible for this decrease in atmospheric pressure with height:
    (1) decrease in height of air column results in a linear decrease in the atmospheric pressure and
    (2) decrease in density of air with height results in a non-linear decrease in atmospheric pressure

  • Season— With change in season on earth, the water vapour content in the atmosphere also changes. Therefore, the variation of pressure occurs with season change.

  • Temperature— With increasing temperature, the atmospheric density decreases. Therefore, atmospheric pressure decreases with increasing temperature.

Effects of Atmospheric Pressure

  • It is interesting to know that there is such big pressure acting all around us. Have you ever thought that how this pressure is not felt by us? It is not felt by us because our blood also exerts pressure on our body from inside. This pressure of blood balances the pressure of the atmosphere such that the atmospheric pressure is not felt by us.

  • When we travel in an aeroplane, our nose may start bleeding, if the aeroplane is not pressurised properly. This happens because the atmospheric pressure at high altitude is lesser than the blood pressure inside our body. It is this difference of pressure that bursts our capillaries within our nose, making our nose bleed. It is for the same reasons an aircraft should be pressurised properly such that the pressure inside the aircraft remains the same as the normal atmospheric pressure at the ground level. Even astronauts wear space suits to counter the zero pressure that exists in the outer space. Fishes in deep sea water experience more pressure than we feel at land. Hence, their internal body pressure is more than ours. If these fishes are brought out of the water, then their body will burst because of the excess outward pressure of their internal body fluid.

  • If you take a fountain pen to higher altitude, then you will see that the pen starts leaking. This happens because of low atmospheric pressure at high altitude. The ink pressure inside the pen becomes higher that the outside atmospheric pressure and as a result, the pen starts leaking.

  • Sucking from a straw works on the same principle. When you suck the air out of the straw, pressure falls inside the straw. This fall in pressure is compensated by the liquid that is forced up by the atmospheric pressure.

Archimedes’ Principle: An Overview

‘Eureka! Eureka!’ Screaming thus, Archimedes came out of his bathtub and ran straight to his king. A popular legend related to the discovery of the principle of buoyancy ends in this manner.

What is this principle of buoyancy? And why is it so important? If you have wondered about this phenomenon, then the following questions must have arisen in your mind.

What has buoyancy to do with the floatation of bodies in liquids? 

Why does a piece of cork rise back to the surface of water even after you force it harder into water? 

Why does a piece of nail made of steel sink but a ship made of the same material float in water?

Why do you feel lighter while swimming in a pool?

How can Archimedes’ discovery be used in determining the purity of a substance?

Let us go through this lesson to get the answers to all the above questions.


When an object is immersed partially or fully in a liquid, it experiences an upward force. This upward force is known as buoyant force and the phenomenon is called buoyancy.

When an object is immersed in a liquid, its weight seems to be less than its actual weight. The buoyant force exerted by the liquid is responsible for this phenomenon.

Cause of buoyant force

When a body is partially or fully immersed in a liquid, the displaced fluid has the tendency to regain its original position due to gravity. An upward force—called the buoyant force—is, thus, exerted on the body by the displaced fluid.

In equilibrium, the buoyant force is balanced by the weight of the immersed body or the force of gravity acting on it.

The magnitude of the buoyant force acting on the immersed body depends upon two factors. 

  • Volume of the immersed body
  • Density of the liquid

The density of a substance, with respect to the density of a liquid, determines whether the substance will sink or float in the liquid. An iron nail sinks in water because the density of iron is greater than that of water. On the other hand, a cork floats in water as the density of cork is less than that of water. Density is expressed in terms of the volume of a substance. Hence, volume plays a major role in deciding whether a substance will sink or float. Such a relation was given by Archimedes.

Know Your Scientist

Archimedes (287−212 BC) was a Greek mathematician and physicist. According to a legend, he discovered the principle of buoyancy (Archimedes’ principle) while taking a bath. It is said that he was so excited with his discovery that he ran naked in the street shouting ‘Eureka’. 

Apart from this principle, Archimedes made some very important contributions to the fields of mechanics and geometry. He is considered one of the three greatest mathematicians of all time.

Archimedes’ Principle 

Archimedes’ principle states that when a body is immersed wholly or partially in a liquid, it experiences an upward buoyant force of magnitude equ…

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