As humans beings we do work, It is the force that enables us to do any work.

To do anything, either we pull or push the object. Therefore, pull or push is called force.

Example – to open a door, either we push or pull it. A drawer is pulled to open and pushed to close.

Effect of Force:

Force can make a stationary body to get into motion. For example, when children see an empty bottle on the ground, they tend to play with it and set it to a motion by kicking it, i.e. by applying a force.

Force can stop a moving body – For example, by applying brakes, a running vehicle can be stopped.

Force can change the speed of a moving body – By accelerating, the speed of a running vehicle can be increased or by applying brakes the speed of a running vehicle can be decreased.

Force can change the direction of a moving object. For example; By applying force, i.e. by moving handle the direction of a running bicycle can be changed. Similarly by moving steering the direction of a running vehicle is changed.

Force can change the shape and size of an object. For example -– By hammering, a block of metal can be turned into a thin sheet. By hammering a stone can be broken into pieces.

Forces are mainly of two types:

(1) Balanced Forces

(2) Unbalanced Forces

Balanced Forces

If the resultant of applied forces is equal to zero, it is called balanced forces.

Example: In the tug of war if both the teams apply similar magnitude of forces in opposite directions, the rope does not move on either side. This happens because of balanced forces in which resultant of applied forces become zero.

Balanced forces can change the shape and size of an object. For example - When forces are applied from both sides over a balloon, the shape and size of the balloon is changed.

Balanced forces do not lead to any change of state of an object. Balanced forces are equal in magnitude and opposite in direction.

Unbalanced Forces

If the resultant of applied forces is greater than zero the forces are called unbalanced forces. An object in rest can be moved because of applying balanced forces.

Unbalanced forces can have the following impact:

  • Move a stationary object.
  • Stop a moving object.
  • Increase or decrease the speed of a moving object.
  • Change the shape and size of an object.

Laws of Motion:

Galileo said that object move with a constant speed when no forces act on them. This means if an object is moving on a frictionless path and no other force is acting upon it, the object would be moving forever. That is, there is no unbalanced force working on the object.

But practically it is not so. Because to attain the condition of zero unbalanced force is impossible. The Force of friction, force air and many other forces always act upon an object.

Newton’s Laws of Motion:

Newton studied the ideas of Galileo and gave the three laws of motion. These laws are known as Newton’s Laws of Motion.

  • Newton's First Law of Motion - Any object remains in the state of rest or in uniform motion along a straight line until it is compelled to change the state by applying an external force.
  • Newton's Second Law of Motion - The rate of change of momentum is directly proportional to the force applied in the direction of the force.
  • Newton's Third Law of Motion - There is an equal and opposite reaction for every action.

Newton’s First Law of Motion:

Any object remains in the state of rest or in uniform motion along a straight line until it is compelled to change the state by applying an external force.

To understand in detail, If an object is in the state of rest, then it will remain in rest until an external force is applied to change its state. Similarly, an object will remain in motion until an external force is applied over it to change its state. The state of any object can be changed by applying external forces only.

Examples of understanding Newton’s First Law of Motion in Everyday Life:

  1. A person in a bus falls backwards when the bus starts moving suddenly. This happens because the person and bus both are at rest while the bus is not moving, but as the bus starts moving the body of the person has the tendency to remain at rest. Because this person falls backwards; if he is not alert. A similar effect applies when the bus in motion suddenly stops applying sudden break, the person falls forward.
  2. When we hang wet clothes over stands or wire, usually before hanging, many jerks are given to the cloths to get them dried quickly, droplets of water from the pores of the cloth falls on the ground; reducing the amount of water in clothes thus they dry quickly. This happens because, when suddenly clothes are made in motion by giving jerks, the water droplets in it have the tendency to remain at rest and they are separated from clothes and fall on the ground. 

Mass and Inertia:

The property of an object because of which it resists getting disturbed from its current state is called Inertia. Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass. A heavy object will have more inertia than the lighter one.

In other words, the natural tendency of an object that resists the change in state of motion or rest of the object is called inertia.

Since a heavy object has more inertia, it is difficult to push or pull a heavy box over the ground than the lighter one.


Momentum is the power of motion of an object.

The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’.

Therefore, momentum of the object = Mass x Velocity.

Or, p = m x v

Where, p = momentum, m = mass of the object and v = velocity of the object.

Let's understand momentum through some real time examples:

Police and army use a small bullet, yet it is able to kill a person when it is fired from a gun because of its momentum due to great velocity.

A person gets injured in the case of being hit by a moving object, such as stone, pebbles or a moving vehicle on road because of the momentum of the object.

Momentum, Mass and Velocity:

Momentum is the product of mass and velocity (p = m x v) of an object. This means momentum is directly proportional to mass and velocity. Momentum increases with an increase of either mass or velocity of an object.

This means if a lighter and a heavier object is moving with the same velocity, then the heavier object will have more momentum than the lighter one.

If a small object is moving with great velocity, it has tremendous momentum. And because of momentum, it can harm an object more severely. For example, a small bullet having a little mass even kills a person when it is fired from a gun.

Usually, road accidents prove more fatal because of high speed than in slower speed. This happens because vehicles running with high speed have greater momentum compared to a vehicle running with slower speed.

Momentum of an which is in the state of rest:

Let an object with mass 'm' is in the rest.

Since the object is at rest, therefore, it's velocity, v = 0

Now, we know that,

Momentum = mass x velocity

Or, p = m x 0 = 0

Thus, the momentum of an object in the rest, i.e. non-moving, is equal to zero.

Unit of momentum:

The SI unit of mass = kg

The SI unit of velocity = meter per second i.e. m/s

We know that, momentum (p) = m x v


p = kg x m/s

Or, p = kg m/s

Therefore, SI unit of momentum = kg m/s