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Pressure Science class-10



The force acting perpendicularly per unit area is called pressure. The SI unit of pressure is N/m2 or Pascal (Pa).


One Pascal pressure:

The pressure exerted in 1m area when 1 N of force is applied is called one Pascal pressure.


Liquid pressure:

The force exerted by liquid on per unit area of the container is called liquid pressure.


Properties of liquid pressure:

i) Liquid pressure increases with increasing depth.

ii) Liquid pressure does not depend upon the volume of it.

iii) Liquid pressure is independent of the shape of the vessel in which it is kept.

iv) Pressure applied on a liquid is transmitted equally in all direction.


Pascal’s Law:

Pascal’s law states that, “the pressure is equally transmitted perpendicularly to all sides as pressure is applied at a place on a liquid contained in a closed container.”


Applications of Pascal’s Law:

Pascal’s law is applicable for the construction of hydraulic press, hydraulic brake, hydraulic lift, hydraulic cranes and other hydraulic machines.


7 Principle of hydraulic press:

The principle of hydraulic machine states that, “a small force applied on a smaller piston is transmitted to produce a large force on the bigger piston.”


Reasons for using liquid in hydraulic press:

i) Liquid transnits pressure equally and perpendicularly in all directions.

ii) Any liquid can not be compressed. i.e. liquid is almost incompressible.


Uses of hydraulic press:

A hydraulic press is used mainly for the following purposes:

i) For pressing cotton bales and good like quilts, books, paper, metal sheets, etc.

ii) For extracting the juice from sugarcane, sugar beet, fruits, etc.

iii) For squeezing oil out of linseed and cotton seeds.



The upward force exerted by a liquid on a body which is immersed in the liquid is known as the upthrust. It is measured in Newton (N).

Upthrust (U) = W1-W2 Where,

W1 = weight of an object in air

W2 = weight of an object when immersed in the liquid.


The upthrust of a liquid depends on:

i) The density of the liquid in which the body is immersed (U ∝ d).

ii) The size or volume of the body immersed in a liquid (U ∝ V).

iii) The value of acceleration due to gravity (U ∝ g).


The forees produced when a body is float or immersed in a liquid:

i) Upthrust (acting vertically upward)

ii) Weight of a body (acting vertically downward)



The mass per unit volume of a substance is called its density. SI unit of density is kg/m3 and CGS unit is g/cm3.

Density (d) = volume(v)/mass(m)


Relative density:

Relative density of a substance is defined as the ratio of the mass of a certain volume of the substance to the mass of the equal volume of water at 4°C.


Relative density =

(Mass of certain volume of the substance) / (Mass of the same volume of water at 4°C)


Relative density has no unit.


The relation between density of liquid and its upthrust:

Upthrust is directly proportional to the density of the liquid.

i.e. U = Vdg


Archimedes’ Principle:

“When a body is partially or wholly immersed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it.”

So, Upthrust = Weight of displaced liquid


Applications of Archimedes’ Principle:

Archimedes’ principle is used to deign :

i) The ships and submarines.

ii) The hydrometers to find the densities of liquids.

iii) The lactometers to test the purity of milk.

iv) The hot air ballons.

v) It is used in determining the relative density of a substance.


Law of floatation:

An object floating in the liquid displaces the liquid equal to its own weight.

Hence, Weight of floating body = Weight of displaced liquid.



The instrument, which is used to measure relative density of a substance and density of the liquid is called hydrometer.


Types of Hydrometer:

Hydrometer is of two types,

i) Constant immersion hydrometer

ii) Constant weight hydrometer



The hydrometer which is constricted to measure the density of only the milk is called Lactometer.


Functions of hydrometer:

i) To measure the relative deasity of a substance.

ii) To measure the density or purity of milk.

iii) To measure the density of liquid.


Conditions for floatation of an object:

i) If density of a substance is less than that of the liquid.


ii) If the weight of the displaced water is equal to weight of an object.


iii) If the density of liquid and substance are equal, the substance will neither sink nor float. It will stay in equilibrium position.



The surface of earth is surrounded by a layer of air from all sides. This layer of air is called atmosphere. The region of air, which surrounds the earth is called atmosphere. The atmosphere covers water and land of the earth in the form of canopy.


Atmospherie Pressure:

The thrust exerted per unit arca on the earth surface due to column of air, is called the atmospheric pressure on the surface of the earth.

The atmospherie pressure at sea level is called standard atmospherie pressure which is 101300 N/m2 or 760 mmHg. The atmospherie pressure at the top of the Mount Everest is 3x104N/m2. The atmospherie pressure decreases as we go from sea level. Therefore, it is less at the top of Mr.Everest than Chitwan, Nepalgunj and Bhairahawa. Due to the difference in air pressure at different altitudes, the air blows from place to place.


Instruments used to measure air pressure:

a) Manometer : A manometer is an instrment used for measuring the pressure exerted by gas.


b) Mercury barometer : A mercury barometer is an instrument used for measuring the atmospherie pressure. The barometer used in the aeroplane or the mountain climbers are called Aneroid barometer. It is also called Altimeter.


c) Pressure gauge: Used for measuring the air pressure in the tubes of the wheels of vehieles such as truck, bus, car, motorcycle, etc.


Types of Barometers:

The following three types of barometers are commonly used:

i) Mercury barometer

ii) Fortin’s barometer

iii) Aneroid barometer


Uses of Barometer:

A barometer is used for the following purposes :

i) To measure the atmospheric pressure at a place.


ii) For weather forecasting.


iii) As an altimeter to measure the height. An altimeter is an aneroid barometer, but it is used in aircraft to measure its altitude.


Water cannot be used instead of mercury in a barometer because of the following reasons:

i) The density of water is less than that of mercury. Therefore, if water is used in a barometer, the height of the tube should be about 11m. It makes the barometer inconvenient to handle.


ii) Water is sticky to the walls of a barometer.


iii) Water is transparent, thus it is not clearly seen in the column.


iv) Water vaporizes easily.


Some instruments based on atmospheric pressure:

i) Syringe

ii) Air Pump

iii) Water Pump


Importance of atmospheric pressure:

i) Atmospheric pressure balances the pressure in and out of our body. So, we are alive. In the absence of atmospheric pressure we can not survive.


ii) Due to atmospheric pressure ink can be filled in pens.


iii) Due to atmospheric pressure a syringe can work. i.e. medicine can be filled in a syringe.


iv) Due to atmospheric pressure water pumps work. i.e. It is important to lift the water by using a water pump.


v) Due to atmospheric pressure we can suck cold drinks and juices like fruit, cocacola, etc. from their containers.


vi) Due to atmospheric pressure air pumps can fill air tubes of ball and different vehicles.


vii) Due to the difference in pressure at different altitudes, the air blows from place to place. So, it causes wind and it also helps to rain.





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Force – Science class-10



A force is a physical quantity which causes or tends to cause a motion in an object at rest or changes or tends to change the direction of motion of a moving object or the shape or size of the object. The SI unit of force is Newton (N) while its CGS unit is dyne (1N=105 dyne).



All the objects in the universe attract each other with a force, which is called gravitation.


Gravitational Force:

The magnitude of force of attraction between any two bodies in the universe is called gravitational force. Its symbol is F and SI unit is Newton (N).


Consequences of gravitational force:

i) Tides in seas and oceans are due to the gravitational force of the sun and moon.


ii) The planets revolve around the sun due to the presence of gravitational force.


iii) Existences of solar system and galaxies.


iv) Rainfall and snow fall is possible only due to gravitational force of the earth.


v) Artificial and natural satellites revolve around the earth due to the gravitational force between the earth and the satellite.


Affecting factors for gravitational force:

i) The product of mass of object i.e., F ∝ M1M2

ii) The square distance between the centres of body i.e., F ∝ 1/d2


Newton law of gravitational:

Newton’s universal law of gravitation states that, “The gravitational force produced between any two bodies of the universe is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.”


Applications of Newton’s Universal law of Gravitation:

i) This law helps to determine the mass of the earth, moon, sun and other heavenly bodies.


ii) This law helps to determine the distance between bodies existing in the universe.


iii) This law helps in discovering new planets, stars and other heavenly bodies.


iv) It is used to study the binary stars.


Verification of Newton’s law:

Consider the two bodies of the mass M and M with force F acting between them towards their centre. If the distance between their centers is d then,


According to Newton’s universal law of gravitation,


We have F ∝ M1M2 —1


F ∝ 1/d2 —2


Combining 1 and 2 we get, F ∝ M1M2/d2




Where G is proportionality constant which is known as universal gravitational constant.

The numerical value of gravitational constant (G) is 6.67*10-11 and its unit is Nm2/Kg2


Gravitational constant ‘G’:

The force of attraction between two bodies having mass 1 kg of each separated by distance 1 m taken from their centre’s is called gravitational constant. Its value is equal to 6.67×10-11 Nm2/kg2 and SI unit is Nm/kg. Its value was calculated by Henry Cavendish by using a sensitive balance called the Torsion balance in 1798 AD.


Conditions for Gravitational force ‘F being equal to Gravitational constant ‘G’:

The gravitational force ‘F’ exerted between two bodies having mass 1 kg each, separated by distance 1m taken from their centers is equal to gravitational constants ‘G’.


Properties of Gravitational constant (G) that makes it universal:

i) The value of ‘G’ is independent of the nature and chemical composition of the masses of bodies.

ii) The value of ‘G’ is independent of the medium (denser or rarer) in which the bodies are kept.

iii) The value of ‘G’ is independent of physical factors like state of object, temperature, pressure, etc.


Null point:

The space between the two heavenly bodies at which the resultant gravity is zero, is called null point.



The force of attraction with which a planet, satellite or star pulls a body towards its centre is called gravity. Its SI unit is Newton (N).


Effects of gravity:

i) Acceleration is produced on freely falling body.

ii) Everybody has some weight.

iii) The earth is surrounded by the atmosphere. Acceleration is produced on freely falling body.

iv) We can stand and walk on the surface of the earth.

v) All the objects like buildings, bridges, etc. can stand on earth’s surface and rivers flow from high level to low level.

vi) Every objects falls towards the surface from a certain height.


Affecting factors for gravity:

i) Mass of planet or satellite (M): Gravity is directly proportional to mass of planet or satellite i.e., F ∝ M.


ii) Radius of planet or satellite (R) : Gravity is inversely proportional to radius of planet or satellite i.e., F ∝ 1/R2.


Gravitational field:

The space around the heavenly body (planet or satellite) up to where it can exert gravitational force on other masses is called gravitational field. Theoretically, the gravitational field extends up to infinity.


Gravitational field intensity (I):

The gravitational field intensity at a point on space around a heavenly body (planet or satellite) is defined as the gravitational force experienced by an object of unit mass placed at that point. It is denoted by ‘I’ and its SI unit is N/kg.


Acceleration due to gravity:

The acceleration produced in a freely falling body due to the force of gravity of the earth is called acceleration due to gravity. It is denoted by ‘g’ and its SI unit is m/s2.


The relation between radius (R) and acceleration due to gravity (g) :

The acceleration due to gravity is inversely proportional to the square of the radius of planet or satellite.


g ∝ 1/R2


Verification of Relation between acceleration due to gravity and radius of the earth:

Let M be the mass and R be the radius of the earth and m be the mass of the body kept at the surface of the earth,

According to the Newton’s law of gravitation the force of attraction between them is given by,


F=G Mm/R2 —1


Also the body is attracted towards the centre of the earth with a force given by,


F=mg —2


From 1 and 2, mg = G Mm/R2


g=GM/R2 —3


G and M are constant whereas R varies because radius of the earth is more at the equator than at poles.


From 3,

We can say that the acceleration due to the gravity is independent to the mass of the body but depends on the mass and radius of the earth.


Variation of the value of g:

a. Variation due to the shape of the earth: g ∝ 1/R2

Value of the g maximum at poles but minimum at equator.


b. Variation due to the height from the surface of the earth: g’ = (R/R+h)2g

If we increase the height from the surface of the earth the quantity in the bracket becomes less than 1. And the acceleration due to the gravity decreases as height from the surface is increased.


c. Variation from the depth of earth surface: g’ = g(R-x)/R since the quantity (R-x)/R is less than 1 .The value of g insidethe earth decreases with increase in depth.

At centre of the earth x=R so gravity at the centre of the earth is 0.


Factors upon which acceleration due to gravity depend:

i) Mass of planet or satellite (M).

ii) Radius of planet or satellite (R).



Total quantity of matter present in any object is called mass. Mass of an object depends on total number of atoms (particles) and average mass of atom. Mass of an object does not change anywhere on the earth, in the space, or on any planets. The SI unit of mass is kilogram (kg).



The force with which the earth attracts any object on its surface towards its centre is known as weight. Its SI unit is Newton (N). Spring balance measures the weight of an object. Weight of an object differs from place to place because of the difference in the value of ‘g’ form place to place.


Free fall:

If an object is falling with the acceleration of acceleration due to gravity in the absence of air resistance is called free fall. The acceleration produced on an object during the free fall is equal to the acceleration due to gravity at that place. There is no perfect free fall of the object falling on the earth due to air resistance. The surface of the moon is not covered by the air. So, the falling of any object on it is free fall.



The weight of an object falling freely under the effect of gravity appears to be zero. It is called weightlessness.


Conditions for weightlessness:

i) When a body is falling freely, it becomes weightlessness.

ii) When a body is in the space at null point.

iii) The body in the rocket (artificial satellite) becomes weightlessness when the rocket (artificial satellite) is orbiting around a heavenly body.


Conclusion of Feather and Coin experiment:

The acceleration produced due to gravity in all object will be the same at a particular place for all, if there is no any external resistance.




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