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A substance which begins to flow under an external force is called a fluid. Liquids and gases are fluids.


The ratio of mass to the volume of a body is called its density. (i.e. mass present in its unit volume). It is a scalar quantity having SI unit kg/m3.
EXAMPLE: The density of water is 1000 kg/m3. The density of water is maximum at 4°C.


It is an instrument used to measure the density or relative density of a liquid. Its working is based on the law of floatation.



Thrust (the normal force) exerted by a liquid per unit area of the surface in contact at rest, it is called fluid pressure.
Fluid pressure (p) = F/A
Its unit is N/m2 or Pascal (Pa).


The pressure exerted by the atmosphere is called atmospheric pressure. An aneroid    barometer is used to measure the atmospheric pressure and height of a place. Other units of atmospheric pressure are torr and bar.


The pressure exerted anywhere at a point of the confined fluid is transmitted equally and undiminished, in all directions throughout the liquid. Hydraulic lift, hydraulic press hydraulic brakes works on the basis of Pascal’s law.



When a body is partially or wholly immersed in a liquid, an upward force acts on it, which is called buoyant force or upthrust and this property of fluids is called buoyancy. The buoyant force is equal to the weight of the liquid displaced by the submerged part of the body.



When a body is partially or completely immersed in a quid, it loses some of its weight. The loss in weight is equal to the weight of the liquid displaced by the submerged part of the body.



A body will float in a liquid if the weight of the body is equal weight of the liquid displaced by the immersed part of the body. In floating condition, the center of gravity (g) and the center of buoyancy (B) of the floating body must lie on the same straight line. Ice and large icebergs float on water surface as its density (0.92 g/cm3) is lesser than the density of water.

It is easier to swim in sea water than in a river as density of sea water is greater than the density of river water. In sea water, buoyant force is greater than that in river water. The density of human body is less than the density of water but the density of human head is greater than the density of water. Therefore, during swimming a person displaces the liquid with hands and legs and total weight of displaced liquid becomes equal to the weight of the body.




The property of a liquid by virtue of which it tries to minimize its free surface area is called surface tension. The minimum surface area of a given amount of liquid is for spherical shape. Therefore, rain drops are spherical.


The surface tension of a liquid decreases with increase in temperature.

Soluble Impurities

If the impurities are less soluble in liquid, then its surface tension decreases. If impurities are highly soluble in liquid, then its surface tension increases.


The surface tension of a liquid decreases with increase in temperature.


  • Surface tension of a liquid becomes zero at critical temperature.
  • When soap, detergent, Dettol, phenyl, etc. are mixed in water then its surface tension decreases.
  • When salt is added in water, it's surface tension increases.
  • When oil spreads over the surface of water, it's surface tension decreases.
  • When kerosene oil is sprinkled on water, it's surface tension decreases. As a result, the larva of mosquitoes floating on the surface of water die due to sinking.
  • Warm soup is tasty because at high temperature it's surface tension is low and consequently, the soup spreads on all parts of the tongue.
  • Antiseptics like Dettol have low surface tension and therefore it reaches in the tiny cracks of the wound and cleans the germs and bacteria.
  • The surface tension of soap solution in water is less than the surface tension of pure water. Therefore, soap solution cleans greasy strains of clothes better than pure water.



The phenomenon of rising or falling of liquid column in a capillary tube (glass tube of very fine bore) is called capillarity.

1. A piece of blotting paper soaks ink because the pores of the blotting paper serve as capillary tubes.
2. The oil in the wick of a lamp rises due to the capillary action of threads in the wick.
3. The root hairs of plants draw water from the soil through capillary action.
4. To prevent loss of water due to capillary action, the soil is loosened and split into pieces by the farmers.
5. If a capillary tube is dipped in water in an artificial satellite, the water rises up to other end of tube because of its zero apparent weight, how long the tube may be.
6. The action of a towel in soaking up water from the body is due to the capillary action of cotton in the towel.
7. Melted wax, in a candle, rises up to wick by capillary action.



The intermolecular force of attraction acting between the molecules of same substance is called cohesive force. e.g., Intermolecular force of attraction acting between the molecules of water, mercury etc.

The intermolecular force of attraction acting between the molecules of different substance is called adhesive force. e.g., Intermolecular force of attraction acting between the molecules of paper and gum, paper and ink, etc.



The force which opposes the relative motion between different layers of liquid or gases is called viscous force.



  • Viscosity is the property of a liquid by virtue of which it opposes the relative motion between its different layers.
  • Viscosity is the property of liquids and gases both.
  • The viscosity of a liquid is due to the cohesive force between its molecules.
  • The viscosity of a gas is due to diffusion of its molecules from one layer to another layer.
  • Viscosity of gases is much less than that of liquids.
  • There is no viscosity in solids.
  • The viscosity of an ideal fluid is zero.
  • With the rise in temperature, the viscosity of liquids decreases and that for gases increases.
  • Viscosity of a fluid is measured by its coefficient of viscosity. Its SI unit is decapoise (kg/ms) or pascal second. It is generally denoted by η.



According to this law, the viscous force depends upon the coefficient of viscosity, velocity of the moving object and its size.


When a small spherical body falls through a long liquid column its velocity increases gradually but later on it becomes constant, called terminal velocity. The radius of spherical raindrops is very small therefore their terminal velocity is also small, with which they strike the earth’s surface. When a liquid flows through a pipe, its speed is maximum near axis and minimum near the walls of the pipe.


When non-viscous liquid flow through a pipe of non-uniform cross-sectional area in stream-lined flow, (i.e. velocity at every point in the fluid remains constant) then at each section of the tube, the product of the area of cross-section of the pipe and velocity of liquid remains constant, i.e. A × v = constant. Therefore speed (v) of fluid flow becomes faster in a narrower pipe.



If a non-viscous and incompressible liquid is flowing in stream-lined flow then total energy, i.e., sum of pressure energy, kinetic energy and potential energy, per unit volume of the liquid remains constant.
Venturi tube and aspirator pump works on Bernoulli’s theorem.
According to Bernoulli's theorem, with increase in velocity of liquid its pressure decreases and vice-versa.
During storms or cyclones, the roofs of the huts or tinned roofs blown off because wind blows with very high speed over the top of the roof and therefore pressure of air decreases. Due to the pressure difference of air above and below the roof, a lifting force acts on the roof. If it is sufficient to balance the weight of the roof it starts to fly off.





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