Unit of Force

  • Sir Issac Newton (1642-1727) One of the greatest scientists the world has ever seen.
  • He was an English mathematician, physicst and astronomer.  The SI unit of force is named after him.

Contact Forces

  • A force that can cause or change the motion of an object by touching it is called Contact Force.

Magnetic Forces

Non Contact Forces

Magnetic                            Gravitational                        Electro Static

 Force                                      Force                                       Force

Gravitational Force

  • Gravity is not a property of the earth alone. In fact, every object in the universe whether small or large, exerts a force on every other object. This force is also known as the gravitational force.

Electrostatic Force

  • The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
  • This force acts when the bodies are not in contact. The electrostatic force is another example of non contact force.

Solved Problem 1:

  • A liquid gives force of 100N over an area of 2m2 . What is the pressure?







  • Blaise Pascal(1623-1662)
  • One of the greatest scientist of the 17th He was a child prodigy. A French mathematician, physicst, inventor, writer and philosopher. The SI unit of pressure is named after him.

Substituting the values


                    =50 N/m2

Pressure      =N/m2

Pressure Exerted by Liquids and Gases

  • You know that liquids and gases are called fluids. Solids always exert pressure downwards. But the fluids exert pressure in all directions.
  • Deep sea divers wear specially designed suits to protect them from the huge pressure of the water underneath.
  • The walls of the dams are made stronger and thicker at the bottom than at the top of the dam to withstand the huge lateral pressure of water at the bottom.

Atmospheric Pressure

  • The earth is surrounded by air all around. This thick envelop of air is called the atmosphere.
  • The atmospheric air extends up to many kilometers above the surface of the earth.
  • The pressure exerted by this air column is known as the atmospheric pressure.
  • The atmospheric pressure at sea level is approximately 1,00,000 N/m2 of (105N/m2)
  • Why do astronauts were a special dress to go into the space?
  • The blood pressure inside our body would need air pressure outside to keep us safe.
  • When we go above the earth’s atmosphere , the pressure outside is very less. But, he pressure inside our body, is very high.
  • Due to this our body will burst. So, to avoid this astronauts wear a special dress.

Measurement of atmosphere pressure

  • The atmospheric pressure is not the same at all places. It decreases as we go above the earth’s surface.
  • The instrument used to measure the atmosphere pressure is called Barometer.
  • In 1643, an Italian scientist named Torricelli invented the first barometer. It was a mercury barometer.
  • Aneroid barometer and Fortein’s barometer are other instruments used to measure the atmospheric pressure.

Pascal’s Law

  • The pressure applied to an enclosed liquid gets transmitted equally to every part of the liquid.
  • This property was first demonstrated by Pascal and is called Pascal’s law.
  • Hydraulic devices, like earth excavators and car brakes work on the above principle.
  • Friction is the force created whenever two surfaces move or try to move over each other.


          Friction is the force created whenever two surfaces move or try to move over each other.

Factors Affecting Friction:

  1. Mass of the body
  2. Nature of the surfaces in contact

Upthrust and Buoyancy

  • When a body floats or immerses in a liquids, the pressure on the bottom surface is more than that the pressure on the top surface.
  • Due to the difference in pressure, an upward force acts on the body. This upward force is called upthrust or buoyant force.
  • The buoyant force is equal to the weight of the liquid displaced.
  • The buoyant force acts through the centre of gravity of the displaced liquids which is known as centre buoyancy.


  • Archimedes discovered many important principles of statics and hydro statics.


  • He invented the water screw for irrigating the fields of Egypt. He discovered the principle of lever.

Archimedes Principle

  • When a body is immersed in fluid it experiences an apparent loss of weight which is equal to the weight of the fluid displaced.

Relative Density

  • Density of a body is defined as mass per unit volume of the body.

   Unit of density is Kg m-3

Relative density

  • Relative density is defined as the ratio of density of the body to the density of water. It has no unit.
  • An iron piece floats in mercury, but sinks in water. This is because the density of mercury (13600 Kg m-3 ) is greater than the density of water (1000 Kg m-3)
  • A ship made up of iron floats in water. This is because the ship is hollow and contains air.
  • The large space inside the ship enables it to displace a volume to water much greater than the actual volume of iron that was used in the construction.
  • So the weight of water displaced is greater than the weight of the ship.

Laws of floatation

  • The weight of the floating body is equal to the weight of the liquid displaced by it.
  • The centre of gravity of the floating body and the centre of gravity of the liquid displaced are in the same vertical line.

More to Know

  • The density of air is 14 Times greater than that of hydrogen. Then weight of a hydrogen filled balloon is much less than the weight of the air it displaces.
  • The difference between the two weights gives the lifting power of the balloon. Thus hydrogen filled balloon flies high in the air.


  • The laws of flotation are made use of in the construction of hydrometers used for the determination of the specific gravities of solids and liquids.
  • There are two types of hydrometers.
  • The constant immersion hydrometer, in which the weight of the hydrometer is adjusted to make it sink to the same fixed mark in all liquids.
  • The variable immersion hydrometer
  • A common hydrometer used to test the purity of milk by noting its specific gravity is called a LACTOMETER.
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