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When a mug full of water is dipped into the container of water, the water
exerts an upward force on the mug. This upward force (buoyant force) decreases the
net force on the mug and the mug feels lighter.

Thrust: The force acting on an object perpendicular to the surface is called thrust. Its SI unit is Newton(N).

Pressure: It is the thrust exerted per unit area of a body. Its SI unit is Pascal (Pa).

Density: It is the mass per unit volume of a body.

Density = \(\frac{Mass\,of\,body}{Volume\,of\,body}\)

Its SI unit is kg m^-3

Relative density: The relative density of a substance is defined as the ratio of the density of substance to that of water.

It is a ratio and thus has no units.

Relative Density : \(\frac{Density\,of\,body}{Density\,of\,body}\)

It has no unit because it is a ratio.

Archimedes Principle states that when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

We can observe it by the following activity:

● Tie a stone at the end of a spring balance.

● Suspend the stone in air by holding the spring balance and observe the reading of the spring balance due to the weight of stone.

● Now, slowly immerse the stone in water in a container.

● As the stone is lowered in the water, the reading of spring balance decreases.

As we know that the reading of spring balance is due to the weight of the stone. The decrease in reading of the spring balance shows the loss in weight of stone due to some force acting on the stone in upward direction. This upward force exerted by the water is equal to the weight of water displaced by the stone. This approves the Archimedes principle.

Pressure is defined as the thrust per unit area. It is the force applied perpendicular to the surface of an object per unit area.

Its SI unit is Pascal (Pa). 1 Pa=1N m^-2

The principle of floatation states that a floating object displaces a weight of fluid equal to its own weight.

If the density of the object kept on the surface of water is less than that of
water, it floats.

If the density of the object kept on the surface of water is more than the density of
water, it sinks in the water.

The thin strap of school bag has very less area of contact. It is made up thin
and strong string. The lesser area exerts more pressure on the shoulders. Thus, it is
difficult to hold the bag after some time.

When ice is immersed in water, it displaces the same amount of water as the
volume immersed. When the ice floating in water melts, the added volume is same as
the volume displaced before. The added volume is equal to the volume of water
displaced and thus the water level does not change.

The up thrust is equal to the weight of water displaced when the object is
immersed in it. More the volume of submerged part of rigid body, more is the water
displaced and thus more is the up thrust.

When the density of body is equal to the density of the water, the up thrust is minimum
because the object just floats.

The snow-shoes are made large and have larger surface area. Pressure is inversely proportional to the surface area of contact. The large area helps in exerting lesser pressure on the ice by the snow shoes and prevents us from sinking.

The relative density of a substance is defined as the ratio of the density of substance to that of water.

It is a ratio and thus has no units.

Relative density = \(\frac{Density\,of\,substance}{Density\,of\,water}\)

The force acting on an object perpendicular to the surface is called thrust.

The factors on which buoyant force depends are:

• Volume of the liquid displaced
• Volume of the body submerged in liquid
• Density of the liquid
• acceleration due to gravity