14 September 2014

Force and Pressure : Science

Force and Pressure : NCERT / CBSE Revision Notes

Force is a push or a pull on an object. The interaction of one object with other results in a force between the two objects. If a force acts on an object in the direction of its motion, then the object moves faster. If a force acts on an object opposite to the direction of its motion, then the objects slows down.
Force also changes the direction of motion of an object. The effect of force changes with the direction of its application. Forces acting on an object in the same direction add to each other, while forces acting opposite to each other results in the difference of the forces.

Force acting on a body can change its shape, as in the case of a stretched or compressed spring. The international unit used to measure force is newton. A force can change the shape and size of an object. There are many types of forces.

Muscular Force

Force and Pressure : Science, CBSE / NCERT Revision Notes, MCQsThe force resulting from the action of muscles is called muscular force. We make use of muscular force of animals like bullocks, horses and camels get our activities done. The point where force is applied on an object is called the point of application of force (Point of contact).


Friction is another type of contact force that always acts opposite to the direction of motion of the objects.

Magnetic force and electro-static force are non-contact forces as they act on an object from a distance.
The force of gravity is also a non-contact and an attractive force exerted by the earth on objects, due to which objects fall to the ground. Gravitational force is an attractive force between any two objects in the universe, and is a non-contact force.

If two forces act on an object in the same direction, the net force acting on it is the sum of both the forces.

If two forces act on an object in the opposite directions, the net force acting on it is the difference between the two forces.

We can classify forces as contact forces or non-contact forces.

Contact force

A contact force is a force between two objects that are in contact with each other directly or indirectly.

Types of contact forces are:
  •  muscular force and
  •  frictional force

Non-Contact force

Any force that is exerted without any contact between two bodies is a non-contact force.  
Types of non-contact forces are:
  • Magnetic Force
  • Electrostatic Force
  • Gravitational Force


Pressure is the amount of thrust acting per unit surface area. Thrust is the force acting on an object perpendicular to its surface. Pressure is measured in newton/metre2, which is equal to 1 pascal (Pa). Liquids and gases exert pressure on the walls of the vessel in which they are present.

Air exerts pressure on all objects. The pressure exerted by air is called atmospheric pressure. A rubber sucker sticks to the surface of an object as atmospheric pressure acts on it. It sucks out all the air from a closed container, creating a vacuum in it.

In order to fix nails to walls, sharp nails are used, as the area of contact of the nail with the wall is small and more pressure acts on it for a given force.
  • The lesser the surface area, the more the pressure exerted by the applied force.
  • Pressure is calculated by dividing thrust by area; Pressure = Thrust/Area
  • Matter that has a tendency to flow is termed as a fluidAll fluids exert pressure.
  •  Liquids exert equal pressure on all the sides of a container at the same level.
  • The Liquid pressure depends on the height of the liquid column.
  • The pressure exerted by a liquid increases with the increase in the height of the liquid column. 
  • The thick blanket of air that covers the earth is termed atmosphere.
  • The pressure exerted by the atmosphere is called atmospheric pressure.
  • The pressure inside our bodies balances the atmospheric pressure around us.