Class 9 Science Notes on Chapter 6 How Forces Affect Motion: Easy Guide for Revision

In our daily life, we observe objects starting, stopping, speeding up, slowing down, or changing direction. Class 9 Science Notes on Chapter 6 How Forces Affect Motion explain the fundamental ideas behind force and motion. This chapter helps students learn the scientific reasons behind these changes and how forces influence the motion of objects around us.

These notes cover important concepts such as force, its magnitude and direction, balanced and unbalanced forces, friction, and the laws that govern motion. The chapter also introduces Newton’s First, Second, and Third Laws of Motion, along with their practical applications in sports, transportation, and daily life.

In addition, the notes explain the relationship between force, mass, and acceleration, the concept of action and reaction forces, and how Newton’s laws can be applied to systems of connected objects. With concise explanations, key formulas, and real-life examples, these notes provide a quick and effective way to revise the chapter and strengthen conceptual understanding before Class 9 assessments.

Important Topics Covered in Class 9 Science Notes on Chapter 6 How Forces Affect Motion 

Introduction to Force	Measuring the Magnitude of Force
Balanced and Unbalanced Forces	Force of Friction
Newton’s First Law of Motion	Newton’s Second Law of Motion
Applications of Newton’s Second Law	Newton’s Third Law of Motion
Applications of Newton’s Third Law	Forces Acting on a System of Objects

Complete Class 9 Science Notes on Chapter 7 How Forces Affect Motion 

Motion describes how an object's position changes with time, but it does not explain what causes that motion. To understand why objects start moving, stop moving, speed up, slow down, or change direction, we study the concept of force.

A force is a push or pull acting on an object. It can change the state of rest or motion of an object, alter its speed or direction, and even change its shape. The study of force helps explain the laws that govern the motion of objects.

The Concept of Force

A force is a push or pull that can affect the motion or shape of an object. For example, kicking a football makes it move, a cricket bat changes the direction of a ball, and squeezing a lemon changes its shape.

Force can,

• Make a stationary object move.
• Stop a moving object.
• Change the speed of an object.
• Change the direction of motion.
• Change the shape or size of an object.

Force is a Vector Quantity

Force has both magnitude and direction. Therefore, force is a vector quantity, just like displacement, velocity, and acceleration.

When describing a force, both its strength and direction must be specified. Changing either the magnitude or the direction of a force changes its effect on an object.

The SI unit of force is the newton (N).

A larger force has a greater effect on the motion or shape of an object than a smaller force.

Measuring the Magnitude of a Force

The magnitude of a force can be measured using a spring balance. A spring balance measures the force applied to its spring by observing how much the spring stretches.

When an object is attached to a spring balance, the reading shows the force acting on the object. For example, the weight of an object is the gravitational force exerted by the Earth, and it can be measured using a spring balance.

Spring Balance

A spring balance is a device used to measure the magnitude of a force or the weight of an object.

Working,
When a force is applied, the spring inside the balance stretches. The amount of stretching indicates the magnitude of the force.

Balanced and Unbalanced Forces

In everyday situations, more than one force usually acts on an object at the same time. The combined effect of these forces determines whether the object remains at rest or changes its motion.

Balanced Forces

Balanced forces are forces that are equal in magnitude but opposite in direction. Since they cancel each other, the net force on the object is zero.

When balanced forces act on an object, 

• The object remains at rest if it is already at rest.
• A moving object continues its motion without any change in speed or direction.

Unbalanced Forces

Unbalanced forces occur when the forces acting on an object are unequal. In this case, the net force is not zero.

When unbalanced forces act on an object:

• A stationary object may start moving.
• A moving object may speed up, slow down, or change direction.

Example:  In a tug of war, if one team pulls harder than the other, the rope moves in the direction of the larger force.

Net Force

The combined effect of all forces acting on an object is called the net force.

Situation	Net Force
Opposite forces of equal magnitude	Zero
Opposite forces of unequal magnitude	Difference of the forces
Forces acting in the same direction	Sum of the forces

Force of Friction

Friction is a force that opposes the motion of an object. It acts between two surfaces that are in contact and always acts in a direction opposite to the motion or the tendency of motion.

When a force is applied to an object, friction resists the movement. The object starts moving only when the applied force becomes greater than the force of friction.

Effects of Friction

Friction can:

• Oppose the motion of an object.
• Slow down a moving object.
• Bring a moving object to rest.
• Prevent objects from slipping.

Friction and Motion

When a box is pushed on a floor, friction acts opposite to the direction of the applied force. If the applied force is greater than friction, the box moves. If the applied force is removed, friction gradually slows the box down and eventually stops it.

Similarly, a bicycle stops after some time when pedalling is stopped because friction acts against its motion.

Other Forces Acting on a Moving Object

Apart from friction, other forces may act on an object placed on a surface.

Force	Direction
Gravitational Force (Weight)	Downward
Normal Force	Upward, perpendicular to the surface
Frictional Force	Opposite to motion

The weight and normal force are usually balanced.

Friction Depends on Surface Nature

The magnitude of friction depends on the nature of the surfaces in contact.

• Rough surfaces produce greater friction.
• Smooth surfaces produce less friction.
• Objects move a shorter distance on rough surfaces.
• Objects move a longer distance on smooth surfaces.

For example, a stack of coins travels farther on a polished marble floor than on a wooden surface because the friction is smaller on the smoother surface.

Newton’s First Law of Motion

Newton’s First Law states that:

An object at rest remains at rest, and an object in motion continues to move with constant velocity unless acted upon by a net external force.

Important Key Points to remember 

• A change in motion occurs only when a net force acts on an object.
• If the net force is zero, the object's acceleration is zero.
• Objects naturally resist changes in their state of motion. This property is called inertia.
• In the absence of friction, a moving object would continue moving forever with constant velocity.

Newton’s Second Law of Motion

Newton’s Second Law explains how force affects the motion of an object.

When a net force acts on an object, it accelerates in the direction of the force. The acceleration is directly proportional to the force and inversely proportional to the mass of the object.

$a = \frac{F}{m} or F=ma$

Where F is the net force (N), m is mass (kg), and a is acceleration (m/s²).

SI unit of force = Newton (N)

1 Newton is the force required to produce an acceleration of 1 m/s² in a 1 kg object.

$1N = 1kg \times 1m/s^2$

Gravitational Force

The force with which Earth attracts an object is called gravitational force.

F=mg

Where m is the mass of the object and g is the acceleration due to gravity.

Value of g

• $g = 9.8\, m/s^2$
• For calculations, g \approx 10\, m/s^2

Applications of Newton’s Second Law

• Catching a Cricket Ball: Fielders move their hands backward while catching a fast ball. This increases the stopping time, reducing acceleration and force, which helps prevent injury.
• Airbags in Vehicles: Airbags increase the time taken to stop the passenger during a collision, reducing the force on the body and lowering the risk of injury.
• Breaking a Coconut: A coconut cracks when it hits a hard surface because it stops in a very short time, producing a large force.

Newton’s Third Law of Motion

Newton’s Third Law explains that forces always occur in pairs during an interaction between two objects.

Whenever one object exerts a force on another object, the second object simultaneously exerts an equal and opposite force on the first object.

Important Key Points to remember 

• Forces always occur in action–reaction pairs.
• The two forces are equal in magnitude and opposite in direction.
• Action and reaction forces act on different objects, so they do not cancel each other.
• Both forces exist at the same time.

Examples of Newton’s Third Law

• Kicking a Ball: When your foot pushes the ball forward, the ball pushes your foot backward with an equal force.
• Walking and Running: While walking, your feet push the ground backward. The ground applies an equal forward force on your feet, helping you move forward.
• Moving a Bicycle Without Pedalling: When you push the ground backward with your feet, the ground pushes you and the bicycle forward.
• Climbing a Tree: A climber pushes the tree trunk downward with their legs. The trunk exerts an equal upward force, helping the climber move upward.
• Rowing a Boat: The paddle pushes water backward, and the water pushes the paddle and boat forward.
• Rocket Launch: A rocket expels gases downward at high speed. The gases exert an equal and opposite force on the rocket, pushing it upward.

Forces Acting on a System of Objects

When two or more objects are connected and move together, they can be treated as a single system. This makes it easier to analyse their motion using Newton’s laws.

Important Key Concepts to remember 

• A system consists of two or more connected objects.
• Internal forces act between objects within the system and cancel each other.
• External forces act from outside the system and determine its motion.
• Only external forces are considered when calculating the acceleration of the entire system.

Acceleration of a System

For two connected objects of masses m₁ and m₂ pulled by an external force F:

$a=\frac{F}{m_1+m_2}$

Where F is the external force, m₁ + m₂ is the total mass of the system, and a is the acceleration of the system. 

Example: Two boxes connected by a string are pulled on a frictionless surface.

• Tension in the string is an internal force.
• The pulling force F is an external force.
• The entire system accelerates as if it were a single object with mass m₁ + m₂.