December 28, 2015


1. Introduction:

When we look around the world we see actually 3 different kinds of things – Solids, Liquids and Gases.

What is the difference between solids, liquids and gases?

Solids have a definite shape, are reasonably rigid i.e. don’t change shape too easily.

Liquids do not have a definite shape and takes the shape of the container.

Gases are even more free. They fill the container fully.

Why do solids, liquids and gases have their nature.

If we keep ice in open air, it melts and becomes a liquid! After some more time, it becomes vapour and vanishes! Surely a solid is not all one piece. If it is becoming a liquid, there is an internal nature it has that gets changed. The same thing with liquid becoming gas.


There is an internal nature to everything.

During the 19th century an extraordinary fact came to light.

Everything, an inch of any material, is made up of crores and crores of small particles called atoms or combinations of atoms called molecules.

The first indication came from experiments in chemistry and later many many experiments gave a detailed account of the inner working of atoms.

The story of the atomic structure is explained fully in the section of “understanding atomic structure”. In a solid the atoms are bonded tightly and hence outwardly, it is solid!

In a liquid the atoms are not in one place but are bonded!

In a gas the atoms are free!

This is the reason why a solid is a solid, a liquid is a liquid and a gas, a gas! When heated, the bonds between the atoms in a solid become weak and becomes a liquid outwardly. On further heating, the bonds become completely broken and a gas is formed. We discuss change of state in the section of “understanding heat”.

In this section we try to gain a deeper understanding into liquids. Liquids and solids together are called fluids.

Pascal’s law, Archimedie’s Principle, and Bernoullis principle explain the deeper working of liquids. Bernoulli’s principle applies to gases also and will be considered in the next section on, Fluids (2) – gases.

2. Pascal’s Law

Pascal’s Law states that if you apply a pressure at one point in a liquid, it spreads equally in all directions.

What does this mean?

A liquid is a liquid. This means that it does not resist. If you apply a pressure at one point, obviously it will get transmitted to all points in the container and in all directions. It is because a liquid does not resist a force that this happens.

Pressure is force per unit area.

Why do we need a concept like pressure for liquids (gases too) and not just force? Liquids do not have a definite shape. It takes the shape of the container. So we cannot use the concept of force. The effect of the force depends on the largeness of the surface on which force acts.

If the area of the surface is large the effect of force would be less. If the area is small, the effect of force would be more.

Thus both force and area are important and this is pressure, the force per unit area does give the pressure.

If the force is 8 Newton and the area in which the force is acting is 4m2, then the effect of force i.e.

Pressure = 8/4 = 2N/m2

If the force is halved (4 Newton) and the area is halved (2m2)

Pressure = 4/2 = 2N/m2

Both factors force and area of contact comes in to play to determine pressure.

Now, Pascal’s law can be used in a dramatic way in a hydraulic press.

Consider the figure below…

If a force is applied at A, a small force, for a long distance l, it gets transmitted to B, a great force for a shorter distance D,

So f x d = F x D (Pressure is the same on both the surfaces)

Thus a small force for a greater distance causes us to lift heavy objects by giving a greater force with a smaller distance.

3. Archimedes Principle

Archimedes was a Greek and he is famous for having discovered a principle that governs all liquids – the Archimedes principle. Let us proceed how to understand his principle.

It is a common observation that when you push a body into a bucket of water, you experience a force in the opposite upward direction. This means that the water exerts a force on you when you immerse a body in water.

If you just put the body in water, it either floats or sinks.

Now why does a body thrust into water or every liquid experience an upward, resistant force?

Why do some bodies float and some bodies sink?

These questions are answered by the Archimedes Principle. When you put a body in water, that body displaces water. The l….. of the water rises. How much water is displaced?

Obviously the volume that the body has occupied in the water, that much of volume of water is displaced. See the figure 2 below.

Now, the displaced amount of water tries to get back into its on final position and thus gives an upward force to the body. This is called the Buoyant force as it is trying to keep the body up.

Obviously this buoyant force is equal to the weight of water displaced.

This is the Archimedes principle.

Now, when does a body sink?

When the weight of the body is greater than the upward force which is nothing but the weight of water displaced, the body obviously sinks.

When the weight of the body is less than the upward force, the body obviously floats.

So clearly 2 things are important here.
The weight of the body itself.
The amount of water (or liquid) the body displaces.

If ‘2’ > ‘1’, the body floats. When ‘2’ < ‘1’ the body sinks. A work floats on water because its weight is less than the amount of water displaced (which gives an upward thrust).

An iron nail sinks on water because its weight is more than the water it displaces i.e., the upward thrust given to the nails greater than the weight of the nail. But what about an iron ship? Why does an iron ship float and not sink in water?

The ship is heavy but it is big in terms of volume also. When it enters water, the amount and hence weight of water it displaces is also huge and the upward thrust is huge!

The upward thrust in the ship is actually more than the weight of the ship! Thus the ship floats!

Thus we see both the factors acting always when we consider floating / sinking
Weight of displaced water that determines the upward thrust on the body.
Weight of the body itself.

‘1’ > ‘2’ – body floats.

‘1’ < ‘2’ – body sinks.

All bodies whether they are floating or into water experiences thrust upward thrust. This is equvalent to weight loss.

The weight acting downward is contracted by a buoyant force upwards.

Apparent weight loss = weight of body – upward thrust (buoyant force)

Thus we clearly see how (and why) bodies behave the way then do when immersed in liquids.