Class 8 Science

Force and Pressure

Force: A push or pull on an object is called force.

Effect of Force on State of Motion

A force can change the state of motion. Both rest and motion are called states of motion. When an object is at rest, its speed is zero.

Effect of force on an object at rest

A force can move an object at rest. For example; if a force is applied on a book the book can move in the direction of force.

Effect of force on a moving object

A force can have following effects on a moving object.

Effect of Force on Shape: Force can change the shape of an object. When force is applied on a ball of dough, there is a change in the shape of dough. Similarly, a potter changes a lump of clay into pots by applying force. When air is filled in a balloon; the shape of the balloon changes because of the applied force.

Types of Force:

There are two main types of forces, viz. contact force and non-contact force.

Contact Force

Contact force is a force which can produce its effect only by making a contact with object. Following are the contact forces:

  1. Muscular Force: It is the force applied by muscles of humans or animals. A bullock is able to pull a cart because of muscular force. The bullock needs to be harnessed to the cart for applying the force.
  2. Friction: When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction. Friction always opposes motion because it acts in opposite direction to motion. Force of friction arises due to contact between two surfaces. If there is no contact between two surfaces there would be no friction.

Non-contact Force

The non-contact force is a force which can produce its effect without making a contact with object. Following are the non-contact forces:

  1. Magnetic Force: The force applied by a magnet is called magnetic force. A magnet can exert its force on an object even from a distance and does not need to touch that object.
  2. Electrostatic Force: It is the force applied by a charged body on another charged or non-charged body. When you brush a comb in your hairs and then bring the comb near small bits of paper, the comb attracts the bits of paper. This happens because of electrostatic force.
  3. Gravitational Force: Every object in the universe exerts a force on another object. This force is called gravitational force. It is the gravitational force of various celestial bodies which keeps them in their position in the space.


Pressure is the force acting on a unit area on an object.

Pressure = Force ÷ Area on which it acts

In this equation, force is the numerator and area is the denominator. This means that pressure is directly proportional to the applied force but inversely proportional to the area on which it is acting.

Following examples illustrate this effect:

Pressure Exerted by Liquids and Gases

Liquids and gases exert pressure as follows:

Atmospheric Pressure: The pressure exerted by atmospheric air is called atmospheric pressure. The weight of air in a column of height of atmosphere and area 10 x 10 cm is 1000 kg. This is roughly same as the area of our head. Can you believe that you have 1000 kg of air on your head?