CBSE – 8th Standard Science

CBSE - 8th Standard Science

Force And Pressure


  • Actions usually results in some kind of change in the motion of an object.
  • 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

  • An interaction of one object with another object results in a force between the two objects.

Exploring Forces

  • Forces applied on an object in the same direction add to one another.
  • If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces.
  • Net force on an object is zero if the two forces acting on it in opposite directions are equal.
  • A force could be larger or smaller than the other. The strength of a force is usually expressed by its magnitude.

 Force can Change the State of Motion

  • A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion. Thus, a force may bring a change in the state of motion of an object.
  • The application of force on an object may change its shape.

State of Motion

  • The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed. An object may be at rest or in motion ; both are its states of motion.
  • An object cannot move by itself, it cannot change speed by itself, it cannot change direction by itself and its shape cannot change by itself.

Contact Forces

 Muscular Force

  • The force resulting due to the action of is known as the muscular force. Animals also make use of muscular force to carry out their physical activities and other tasks.
  • Since muscular force can be applied only when it is in contact with an object, it is also called a contact force.


  • The force responsible for changing the state of motion of objects is the force of friction.


  • The force of friction always acts on all the moving objects and its direction always opposite to the direction of motion. Since the force of friction arises


Non-contact Forces

Magnetic Force

  • 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 so the force exerted by a magnet on a piece of iron is also a non contact force.

Electrostatic Force

  • 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 they are not in contact. The electrostatic force, is also non-contact force

Gravitational Force

  • Objects or things fall towards the earth because it pulls them. This force is called the force of gravity. This is an attractive force it acts on all objects. 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 ,Forces act perpendicular to the surface on which the pressure is to be computed.
  • The area is in the denominator So, the smaller the area, larger the pressure on a surface for the same force.


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.

Atmospheric Pressure

Air is 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.

Liquids and gases exert pressure on the walls of their container