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Force-pressure-theory Physics-day-3 Target ssc cgl 2018

Force-pressure-theory Physics-day-3 Target ssc cgl 2018


In science, a push or a pull on an object is called a force. Thus, we can say that the motion imparted to objects was due to the action of a force .Forces are due to an Interaction of one object with another object results in a force between the two objects. A force

  • May make an object move from rest.
  • May change the speed of an object if it is moving.
  • May change the direction of motion of an object.
  • May bring about a change in the shape of an object.
  • May cause some or all of these effects.

Newton’s Laws of Motion

I law

Every body continues to be at rest or in uniform motion unless an external force is applied to it.

This is also called law of inertia.

Inertia :  Inertia is the property of a body by the virtue of which the body opposes change in its initial state of rest or uniform motion on a straight line. It is of two types..

  • Inertia of rest
  • Inertia of motion

Newton’s I law defines force.

Force : Force is an external effect on the body which changes or tries to change the initial position of rest or uniform motion of the body.

II law

The rate of change of momentum of a body is directly proportional to the force applied on the body.

Force = mass * acceleration

III law

To every action, there is an equal and opposite reaction.

Some important definitions

Momentum : The product of mass and velocity is called momentum.

Momentum = mass * velocity

Impulse :  A large force acting on a body for a very small time is called impulse.

Centripetal force : When a body moves in a circular direction, a force acts on it continuously towards the centre of the path.  Therefore, some external force is required to maintain the circular motion of the body. This external force is called the centripetal force.

Centrifugal force : Centrifugal force is a pseudo force which is equal and opposite to centripetal force.


The force responsible for changing the state of motion of objects in all these examples is the force of friction. It is the force of friction between the surface of the ball and the ground that brings the moving ball to rest. The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion. Since the force of friction arises due to contact between surfaces, it is also an example of a contact force.

Non-contact Forces

A magnet can exert a force on another magnet without being in contact with it. The force exerted by a magnet is an example of a non-contact force Similarly, the force exerted by a magnet on a piece of iron is also a noncontact force.

Electrostatic Force 

A straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper. Such a straw is an example of a charged body. The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. This force comes into play even when the bodies are not in contact. The electrostatic force, therefore, is another example of a non-contact force.

Gravitational Force

Objects or things fall towards the earth because it pulls them. This force is called the force of gravity, or just gravity. This is an attractive force. The force of gravity acts on all objects. 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 known as the gravitational force.


The force acting on a unit area of a surface is called pressure.

Pressure = force / area on which it acts.

At this stage we consider only those forces which act perpendicular to the surface on which the pressure is to be computed. So, the smaller the area, larger the pressure on a surface for the same force.

The device used to measure pressure is called barometer.

Pressure exerted by Liquids and Gases

Liquids exert pressure on the walls of the container. Gases, too, exert pressure on the walls of their container.

Pascal’s law for pressure (liquids)

If gravitational attraction is negligible, pressure is same at all points in a liquid.

If an external pressure is applied to an enclosed fluid, it is transmitted undiminished to every direction.

Atmospheric Pressure

You know that there is air all around us. This envelop of air is known as the atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure.

The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.


  • Force could be a push or a pull.
  • A force arises due to the interaction between two objects. Force has magnitude as well as direction. A change in the speed of an object or the direction of its motion or both implies a change in its state of motion.
  • Force acting on an object may cause a change in its state of motion or a change in its shape. A force can act on an object with or without being in contact with it.
  • Force per unit area is called pressure.
  • Liquids and gases exert pressure on the walls of their containers.
  • The pressure exerted by air around us is known as atmospheric pressure.


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