CBSE Class 9 Science Revision Notes Chapter 9

CBSE Class 9 Science Revision Notes Chapter 9 – Force and Laws of Motion

Class 9 Science Chapter 9 Notes give a comprehensive account of the fundamental concepts of Science, such as force, types of force and the laws of motion. Chapter 9 Science Class 9 Notes are prepared by a team of subject matter experts to provide an in-depth explanation of the essential topics in the NCERT books. Extramarks provides Class 9 Chapter 9 Science Notes in PDF format, which you can download from the website.

Class 9 Science Notes Chapter 9 also includes important questions with answers that can be helpful for students in achieving good marks in the final exams. Extramarks provides revision notes for all subjects of Class 9, which includes important formulas and CBSE extra questions. Apart from CBSE revision notes, students can also download and solve CBSE sample papers and CBSE past years’ question papers provided on the Extramarks website. It will help them understand the exam pattern and learn time management skills to score good marks in the exam.

Access Class 9 Science Chapter 9 – Force and Laws of Motion Notes

Class 9 Chapter 9 Science Notes: Balanced and Unbalanced Forces

Class 9 Chapter 9 Science Notes gives a detailed analysis of balanced and unbalanced forces. When the result of the force applied is equal to zero, it is said that a balanced force is applied. For example, in the game of tug of war, force is applied by both teams, and it is equal in magnitude, so the rope does not move in any direction. In such a case, balanced force is applied, and the resultant force becomes zero. 


Here is a quick overview of the characteristics of balanced forces:

  • There can be no change in the object’s state because of balanced forces. 
  • These forces are opposite in direction and equal in magnitude. 
  • Such types of forces can change the shape and size of the object. 
  • The most common example is when a balloon is pressed from the opposite side; its shape and size change. 

If the result of the applied force on an object is greater than zero, then that force is an unbalanced force. 

Unbalanced Force Can:

  • Change the shape and size of an object.
  • Change the speed and position of an object. 

Force and Laws of Motion Class 9 Notes: Types of Forces

The types of forces are as follows:

Muscular Force: These are the types of forces exerted by the muscles in the human body. 

Gravitational Force: It refers to the force that pulls the body down as a result of its attraction to the earth.

Frictional Force: This force helps to oppose a particular object’s motion while it is in contact with another object or surface. 

Air Resistance: The frictional force that the air exerts on a flying object is known as air resistance. 

Class 9 Science Ch 9 Notes: Newton’s Laws of Motion

Newton has given three classic mechanical laws of motion. These laws describe the relationship between an object in motion and the force exerted on it. Let’s review the three laws of Newton. 

First Law of Motion

The First Law of Motion states that any object in a stationary position is in a state of rest. However, any moving object is in a state of motion until an external force is exerted on it. This results in the change of state of the object. It happens due to the property of inertia. Hence, the First Law of Motion is also known as the Law of Inertia.


All objects tend to be in motion or rest until an external unbalanced force is applied. This tendency is known as inertia. Inertia depends on the mass of the body. Therefore, you can say that the object’s mass is its accurate measure of inertia. 

Inertia can be classified into three types:

The inertia of rest – For example, a passenger travelling in a bus leans forward when the driver applies brakes suddenly, fruits fall from the trees when shaken heavily, moving dust particles on a carpet when beaten with a stick. 

The inertia of motion – For example, a man getting down from a train tends to lean in the forward direction. 

The inertia of direction – For example, the water particles sticking to the cycle tyre fly off tangentially when the driver takes a turn, and passengers feel the force away from the centre of the curve. 

Second Law of Motion

The Second Law of Motion states that the acceleration of an object when subjected to an external force is directly proportional to the magnitude of the net force moving in the same direction. The same is inversely proportional to the mass of the object. One Newton force applied is equivalent to the produced acceleration of 1 m/s2 on an object with a mass of 1 kg. Force is also considered a vector quantity. Hence, the Second Law of Motion states the quantitative definition of force. 


The momentum of an object is defined as the product of its mass and velocity. It is a vector quantity, and the direction of the momentum will be the same as the velocity. The alphabet p is used to represent it. The mathematical calculation of momentum is p = mv. Here m is the mass, and v is velocity. The SI unit = kg m/s. 

An example of momentum is a cricket ball and a tennis ball moving with the same velocity. It takes more force to stop the cricket ball than the tennis ball. This is because the mass of the cricket ball is more than that of the tennis ball. Hence, the force required to stop a moving object depends on its mass. 

Concept of System

Before you understand how to conserve momentum, you need to understand the concept of the system. 

  • The part of the universe chosen for analysis is known as the system. 
  • All that is external to the system is referred to as the environment.

Conservation of Momentum

According to Newton’s Third Law of Motion, the action and reaction forces change the velocity of the bodies, which eventually changes their momentum as well. 

  • The total momentum of an isolated system is always conserved. 
  • An isolated system refers to the net external force on the system, which is zero. 

For example, when you fire a bullet from the gun, the gas produced in the barrel exerts a lot more force on the firing bullet. As a result, the bullet moves forward with a high muzzle velocity. The bullet then exerts an equal and opposite force on the gun: the reaction force. Because of this, while firing, the gun moves backwards. This backward movement of the gun is known as recoil. The velocity with which the gun moves in a backward direction is known as the recoil velocity of the gun. 

Third Law of Motion

Newton’s Third Law of Motion states that every action has an equal and opposite reaction. Both forces act simultaneously. Some day-to-day examples of Newton’s Third Law of Motion are as follows:

  • When a person jumps from a diving board, he pushes the board backwards, and the board forces and pushes the man in the opposite direction. 
  • Birds flying high in the sky push back the air from their wings, whereas the air exerts force and helps the bird to move in an upward direction. 
  • A swimmer, while swimming, pushes the water in a backward direction while the water exerts a force on the swimmer and helps him push forward. 

Inertial and Non-Inertial Frames

Newton’s Laws of Motion do not exist in the non-inertial frame of reference. It is a frame of reference where the object undergoes acceleration with the inertial frame. However, Newton’s Laws of Motion have a hold in the inertial frame of reference.

FAQs (Frequently Asked Questions)

1. Why can a small mass such as a bullet kill a person when fired from a gun?

It is so because even if the mass of the bullet is small, it moves out of the gun with a very high velocity, due to which the momentum produced is high (p = mv). This high momentum of the bullet kills a person.

2. Explain the necessary condition to change the shape or position of an object.

The application of force is necessary for changing the shape or position of an object.

3. Explain the relationship between Newton's three laws of motion.

The first law explains the unbalanced force needed to change the body’s position. The second law explains the force required to produce any given acceleration. Furthermore, the third law explains how these forces acting on a body are interrelated.

4. What is the S.I. unit of force?

The S.I. unit of force is Newton.

5. Why is it hazardous to jump out of a moving vehicle?

Our body is in motion in a moving vehicle. On jumping out of a moving vehicle, our lower body comes to rest while we touch the ground, while the upper part of our body stays in motion and keeps moving forward due to inertia. Hence the person can fall, so it is advisable to move in the direction of the moving vehicle.