Introduction
Welcome to the free notes for Class 9 Science Chapter 6 notes – How Forces Affect Motion. This chapter explains the concept of force, its effects on motion, balanced and unbalanced forces, Newton’s laws of motion, inertia, momentum, and their applications in everyday life. These notes are designed for quick revision and exam preparation.
Based on the Latest NCERT Textbook
These notes are prepared according to the latest NCERT Exploration: Textbook of Science for Grade 9 and cover all important concepts in an easy-to-understand format.
Table of Contents
Based on the Latest NCERT Textbook
These notes are prepared according to the latest NCERT Exploration: Textbook of Science for Grade 9 and cover all important concepts in an easy-to-understand format.
What is Force?
A force is a push or a pull acting on an object. It can change the state of rest or motion of an object, change its direction of motion, or change its shape.
SI Unit: Newton (N)
Effects of Force
A force can:
- Start the motion of an object.
- Stop a moving object.
- Change the speed of an object.
- Change the direction of motion.
- Change the shape or size of an object.
Balanced and Unbalanced Forces
Balanced Forces
Balanced forces are equal in magnitude and opposite in direction. They do not change the state of motion of an object.
Example: A book resting on a table.
Unbalanced Forces
Unbalanced forces are unequal and result in a change in the motion or shape of an object.
Example: Kicking a football.
Newton’s First Law of Motion
Newton’s First Law states that an object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an unbalanced external force.
This law is also known as the Law of Inertia.
Inertia
Inertia is the property of an object to resist any change in its state of rest or motion.
Types of Inertia
- Inertia of Rest
- Inertia of Motion
- Inertia of Direction
Newton’s Second Law of Motion
Newton’s Second Law states that the force acting on an object is equal to the rate of change of its momentum.
Formula:
Force (F) = Mass (m) × Acceleration (a)
SI Unit of Force: Newton (N)
Momentum
Momentum is the quantity of motion possessed by an object.
Formula:
Momentum = Mass × Velocity
SI Unit: kg·m/s
Newton’s Third Law of Motion
Newton’s Third Law states:
“For every action, there is an equal and opposite reaction.”
Examples:
- Walking
- Swimming
- Rocket launch
Law of Conservation of Momentum
The total momentum of an isolated system remains constant if no external force acts on it.
Everyday Applications of Newton’s Laws
- Wearing seat belts in vehicles.
- Using airbags in cars.
- Pushing a shopping trolley.
- Launch of rockets.
- Walking and running.
Important Terms
- Force
- Balanced Force
- Unbalanced Force
- Inertia
- Momentum
- Newton’s Laws of Motion
- Action
- Reaction
- Acceleration
Chapter Summary
✅ Force is a push or pull.
✅ Balanced forces do not change motion.
✅ Unbalanced forces change the state of motion.
✅ Newton’s First Law explains inertia.
✅ Newton’s Second Law gives the relationship between force, mass, and acceleration.
✅ Newton’s Third Law states that every action has an equal and opposite reaction.
Complete Your Preparation
The notes above cover only the basic concepts of this chapter.
For complete exam preparation, get our premium study material prepared by Science World by Tushar Sir.
The complete study material includes:
✔ Detailed Notes
✔ Mind Maps
✔ NCERT Solutions
✔ MCQs with Answers
✔ Assertion & Reason Questions
✔ Case-Based Questions
✔ Competency-Based Questions
✔ Short & Long Answer Questions
✔ Practice Paper
FAQ
What is force?
Force is a push or pull that can change the motion, direction, or shape of an object.
What is the SI unit of force?
The SI unit of force is the newton (N).
What is inertia?
Inertia is the property of an object to resist changes in its state of rest or motion.
What is Newton’s Third Law?
It states that for every action, there is an equal and opposite reaction.
Are these notes enough for exams?
These notes are useful for quick revision. For complete preparation with detailed explanations, NCERT solutions, MCQs, case-based questions, mind maps, and practice papers, the complete eBook is recommended.
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