Force - Definition, Types and Unit of Force

What is Force? 

Force is defined as an external cause that changes or tends to change the state of the body once applied, if the body is in motion it comes to rest and if at rest then will come to motion. It can also cause a change in the direction, shape, size, etc of the body. Example: Pushing or pulling a door by applying force.

Force is a vector quantity which means it has both magnitudes as well as direction. According to Newton's second law, force is stated as the “product of mass and acceleration of a body”. 

What are the Effects of Force?

Force acting on the body may affect the shape of the body or will accelerate or decelerate the body, or it will give motion to the static body or it will stop the motion of the dynamic body. 

Effect of Force Can Be Understood By the Following Points - 

  • The force increases the speed of a moving object or it may also move the stationary object. 

  • Force can stop the moving object or it can just decrease the speed of the moving object.

  • The effect of force changes the direction of the object. 

  •  Force changes the shape and size of the object. 

Types of Forces:

There are two types of forces:

1. Contact forces 

2. Non- contact forces 

Contact Force:

Contact force is a force which is applied by actually touching the body.


a. Tension Force

b. Spring Force 

c. Normal Force

d. Air resistance Force

e. Frictional Force 

f. Force of Gravity 

g. Applied Force 

Tension Force: Tension force is that force which is applied on the cable, rope, or string at the ends. It is directed towards the length of the wire and pulls the energy equally from both ends. Mathematically it is shown as: 

T = mg + ma

Where T = tension in Newton (N), m = mass in (kg), 

g = gravitational force (9.8m/s²) 

Spring Force: Spring force is also known as restoring force. If any force acts on the body which will either compress or stretch the body then this force acts to restore the equilibrium position. For example in the case of rubber. Mathematically it is written as: 

F = kx

Where, F = spring force, k = spring constant, 

x = distance stretched or compressed 

Normal Force: The normal force is again a contact force. This force only exists if two surfaces are in contact otherwise it will not act, from the figure, it can be clearly understood. An example is a book lying on the desk, etc.

Air Resistance Force: If an object is moving through the air then this force will occur and it will resist the body from moving. 

In other words, air behaves as friction and it will act in the opposite direction of the body. 

Frictional Force: Frictional force is the opposing force. This force is generated when the two surfaces are in contact or slide opposite to each other. The surface of the body plays a major role in this type of friction. More rough the surface more will be the friction. 

There are Two Types of Friction:- 

  • Sliding Friction stops the body from sliding.

  • Static Friction acts to keep the object in rest 

Force of Gravity: It is the force that pulls the body toward the centre of the earth due to gravity.

Mathematically:  W = mg 

Where W = weight, m = mass of the body and 

g = acceleration due to gravity 

Applied Force: It is clear from its name that a body or person is applying force on the other body that changes the velocity of the body or the direction of the body, etc. 

There are Three Types of Applied Force:-  

  • Push: It is the most common form of force. The body in this is being pushed. Example: Throw a ball etc. 

  • Pull: Pulling an object. For example, pulling the door to open it. · 

  • Drag: It requires two bodies. When one body is dragged over another. 

Non Contact Force

It is the force that acts without any physical contact between the bodies. 

For example, a ball is thrown in the air; it will come back due to gravitational force which is Non-contact force. There are three types of Non-Contact Force. These are: 

  • Gravitational Force 

  • Electrostatic Force 

  • Magnetic Force 

Gravitational Force: Gravitational force is that force which acts between the two bodies having some mass. It is an attractive force. Two bodies having some mass try to pull each other and never push apart. Example - A stone thrown from Burj Khalifa experiences a force of Gravity which is a Non-contact force. 

Electrostatic Force: The electrostatic force acts between two charged bodies. It is very similar to gravitational force but the only difference is that the gravitational force acts between masses whereas this force acts between charged bodies. Example - On rubbing scale on woollen cloth and take it near to the pieces of paper, it will attract those paper. 

Magnetic Force: It is also a type of non-contact force. The force between the iron and magnet is the magnetic force. For example the magnetic poles in the DC motor.

Unit of Force:

Unit of force is Newton (N). A Newton is a force required to give a mass of 1 kilogram (1 kg) an acceleration of 1 meter per second squared (1 m/s²). SI unit of mass is kilogram (kg) and acceleration is meter per second squared (m/sec²) hence it is written as kg m/sec² which is denoted by Newton.

Line of Action:

The application of force can be represented by a line of action. It is the geometric representation of the force.

Line of action is referred to as the characteristic line that is associated with each force. The orientation of this line can be described using a reference axis. Force attempts to rotate the body if the line of action of a force does not pass through the body. This introduced the concept of moment.