Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 6. How Forces Affect Motion

CBSE Notes New Syllabus: Class 9

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Chapter 6. How Forces Affect Motion

Force is one of the most important concepts in Physics because every change in the motion of an object is caused by a force. In our daily life, we push, pull, lift, throw, kick and stop objects using different kinds of forces. This section explains the concept of force, its characteristics, types and effects with classroom-oriented explanations. 

1. Force

A force is a push or a pull acting on an object. It changes the state of rest or motion of an object and may also change its shape or size. 

Definition

Force is a physical quantity that can change the position, speed, direction or shape of an object.

Examples

• Kicking a Football – The ball starts moving because of force.
• Striking a Cricket Ball – The bat changes the direction of the ball.
• Squeezing a Lemon – The force changes the shape of the lemon.

2. Characteristics of Force

Every force has certain important characteristics that help us describe its effect on an object. 

Main Characteristics

• Magnitude – Indicates the strength of the force.
• Direction – Shows the direction in which the force acts.
• Point of Application – The position where the force is applied.
• Nature – Force may be a push or a pull.

3. Force is a Vector Quantity

Force has both magnitude and direction. Therefore, it is called a vector quantity. If either the magnitude or direction changes, the effect of the force also changes. 

Key Points

• Both magnitude and direction are necessary to describe a force.
• Two forces having the same magnitude but different directions produce different effects.
• The direction of force is represented by an arrow.

4. SI Unit of Force

Definition

The SI unit of force is the newton (N). It is named after the scientist Sir Isaac Newton. 

Important Facts

• Unit Name → newton
• Symbol → N
• CGS Unit → dyne
• Force is measured using a spring balance.

5. Measuring Force

The magnitude of force can be measured with the help of a spring balance. A spring balance measures not only the weight of an object but also the magnitude of any applied force. 

Working Principle

• The spring stretches when a force is applied.
• Greater stretching indicates a greater force.
• The scale reading gives the magnitude of the applied force.

6. Types of Force

Forces can be classified into two main categories depending on whether physical contact between objects is necessary.

Classification of Force

• Contact Forces – Require physical contact between objects.
• Non-contact Forces – Act without physical contact.

7. Contact Forces

Definition

Contact forces are forces that act only when two objects touch each other.

Examples

• Muscular Force
• Frictional Force
• Normal Force
• Tension Force

8. Non-contact Forces

Definition

Non-contact forces act even when the objects are not in physical contact.

Examples

• Gravitational Force
• Magnetic Force
• Electrostatic Force

9. Effects of Force

Depending on the situation, a force may produce one or more effects on an object. 

Effects of Force

• Starts Motion – A stationary object begins to move.
• Stops Motion – A moving object can be brought to rest.
• Changes Speed – Speed may increase or decrease.
• Changes Direction – The object may move in a different direction.
• Changes Shape – Objects like rubber balls or lemons change shape.

10. ATP Education Classroom Summary

• Force → A push or pull acting on an object.
• Force is a Vector → It has both magnitude and direction.
• SI Unit → Newton (N).
• Measuring Instrument → Spring Balance.
• Contact Forces → Muscular, Frictional, Normal and Tension Forces.
• Non-contact Forces → Gravitational, Magnetic and Electrostatic Forces.
• Main Effects → Start motion, stop motion, change speed, change direction and change shape.

ATP Education Exam Booster

Remember that every force has both magnitude and direction, making it a vector quantity. Learn the SI unit of force, the spring balance as the measuring instrument, and the difference between contact and non-contact forces. These concepts form the foundation for understanding balanced forces and Newton's laws in the next sections.