CBSE Class 9 Science Revision Notes Chapter 6: How Forces Affect Motion

Force can change the state of rest, speed, direction or shape of an object. CBSE Class 9 Science Chapter 6 explains how net force, friction and Newton’s laws affect the motion of objects.

An object may remain at rest, move with constant velocity or accelerate depending on the forces acting on it. When several forces act together, their combined effect determines whether its motion changes.

These CBSE Class 9 Science Revision Notes Chapter 6 cover force, friction and Newton’s three laws for the 2026–27 session. Use these CBSE Class 9 Science Chapter 6 notes to revise formulas, numerical examples and daily-life applications.

Key Takeaways

• Force: It has magnitude and direction and is measured in newtons.
• Net force: A non-zero net force changes an object’s velocity.
• Newton’s second law: Force, mass and acceleration are related by F = ma.
• Force pair: Action and reaction forces act on two different objects.

Access Class 9 Science Chapter 6 How Forces Affect Motion Notes in 30 Minutes

Revise the chapter in three parts:

• First 10 minutes: Force, balanced and unbalanced forces, net force and friction
• Next 10 minutes: Newton’s first law, inertia, Newton’s second law and force calculations
• Final 10 minutes: Newton’s third law, action-reaction pairs, rockets and systems of objects

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Force and Its Effects in Class 9 Science Chapter 6

A force is a push or pull acting on an object. It can affect both stationary and moving objects.

A force can:

• Set a stationary object in motion
• Stop a moving object
• Increase or reduce speed
• Change the direction of motion
• Change the shape of an object

Kicking a stationary ball makes it move. A cricket bat can change the speed and direction of a moving ball.

Squeezing a lemon changes its shape. The effect depends on the magnitude and direction of the applied force.

Magnitude and Direction of Force

Force has both magnitude and direction.

The magnitude represents the strength of the force. The direction tells us where it acts.

The effect of a force changes when:

• Its magnitude changes
• Its direction changes
• Both magnitude and direction change

Friction acts opposite to motion. Gravitational force acts towards the Earth, while buoyant force acts upwards on an object in a liquid.

SI Unit of Force

The SI unit of force is the newton.

Its symbol is N.

One newton is the force required to produce an acceleration of 1 m s⁻² in an object of mass 1 kg.

1 N = 1 kg m s⁻²

Measuring Force with a Spring Balance

A spring balance measures the magnitude of a force.

It is commonly used to measure an object’s weight. Weight is the gravitational force with which the Earth pulls an object.

Pulling the free end stretches the spring inside the balance. The reading on its scale gives the magnitude of the force.

Balanced and Unbalanced Forces in How Forces Affect Motion

Several forces may act on an object at the same time. Its motion depends on their combined effect.

The ideas of balanced and unbalanced forces help us understand why an object remains still, moves steadily or accelerates.

Balanced Forces

Balanced forces are equal in magnitude and opposite in direction.

When they act on the same object, the net force is zero. They do not change its state of rest or constant motion.

Examples include:

• Two teams pulling equally in a tug of war
• A barbell held steadily by a weightlifter
• A ball floating on water
• A box moving steadily when the applied force equals friction

A floating ball experiences gravitational force downward and buoyant force upward. It stays at the same position when these forces balance.

Unbalanced Forces

Forces are unbalanced when their combined effect is not zero.

A non-zero force can:

• Set an object in motion
• Stop a moving object
• Increase or decrease speed
• Change the direction of motion

In a tug of war, the rope moves towards the team applying the larger force.

Net Force in Class 9 Science Chapter 6

Net force is the combined effect of all forces acting on an object.

The object’s acceleration depends on the magnitude and direction of this net force.

Forces Acting in the Same Direction

When forces act in the same direction, their magnitudes are added.

Net force = F₁ + F₂

Example:

Two forces of 10 N and 6 N act towards the right.

Net force = 10 N + 6 N
Net force = 16 N towards the right

Forces Acting in Opposite Directions

When forces act in opposite directions, subtract the smaller force from the larger force.

Net force = Larger force − Smaller force

Example:

A force of 10 N acts towards the right. Another force of 6 N acts towards the left.

Net force = 10 N − 6 N
Net force = 4 N towards the right

The direction of net force is the direction of the larger force.

Force of Friction and Its Effect on Motion

The force of friction opposes relative motion between two surfaces in contact.

It acts opposite to the direction of motion or attempted motion.

When a box is pushed gently, it may not move because friction balances the applied force. It starts moving when the applied force becomes greater than friction.

Forces Acting on an Object on a Surface

A box placed on a horizontal floor may experience four forces:

• Applied force: Acts in the direction of the push
• Friction: Acts opposite to motion
• Gravitational force: Acts vertically downward
• Normal force: Acts vertically upward

The normal force is applied by the surface on the object.

Weight and normal force may balance each other. Horizontal motion then depends on the applied force and friction.

Why Moving Objects Come to Rest

A ball gradually slows down after it is pushed. A bicycle also stops after the rider stops pedalling.

Friction continues acting against motion. It reduces velocity until the object comes to rest.

This does not mean that force is always needed to maintain motion. In everyday conditions, continuous force is mainly needed to overcome friction.

Effect of Different Surfaces on Friction

Friction depends on the nature of the surfaces in contact.

A rough surface usually produces more friction than a smooth surface.

An object generally travels:

• A shorter distance on a rough surface
• A longer distance on a smooth surface

A spring balance can provide an approximate measure of friction. A larger reading indicates greater friction.

Useful Effects of Friction

Friction also makes several activities possible.

It helps us:

• Walk without slipping
• Hold objects securely
• Write on paper
• Apply vehicle brakes
• Move bicycles and cars forward

Grooves on footwear and treads on tyres improve grip by increasing friction.

Newton’s First Law of Motion and Inertia

Newton’s first law of motion describes the behaviour of an object when no net force acts on it.

It states:

An object at rest remains at rest, and an object in motion continues with constant velocity, unless a net force acts on it.

When net force is zero:

• A stationary object remains stationary.
• A moving object continues in a straight line.
• Its speed remains unchanged.
• Its direction remains unchanged.
• Its acceleration is zero.

Inertia

Inertia is the tendency of an object to resist a change in its state of rest or motion.

A stationary object resists being moved. A moving object resists changes in speed or direction.

Mass is related to inertia. A heavier object generally offers more resistance to a change in motion than a lighter object.

Constant Velocity and Zero Net Force

Constant velocity means that speed and direction remain unchanged.

Several forces may still act on an object. However, if they balance each other, the net force remains zero.

Suppose a person pushes a moving box forward with a force equal to friction. The box continues moving with constant velocity.

Motion in the Absence of Friction

Imagine an object moving on a perfectly smooth horizontal surface.

If friction is absent, the moving object continues with constant velocity. No further force is required to maintain its motion.

A force is needed only to change its speed, direction or state of motion.

Newton’s Second Law of Motion: Force, Mass and Acceleration

Newton’s second law of motion explains how net force produces acceleration.

It states:

When a net force acts on an object, it accelerates in the direction of the net force.

The acceleration:

• Increases when force increases
• Decreases when mass increases
• Acts in the direction of net force

Relationship Between Force, Mass and Acceleration

The relationship between force, mass and acceleration is:

F = ma

Where:

• F = net force
• m = mass
• a = acceleration

It can also be written as:

a = F/m

For the same mass, a larger force produces greater acceleration.

For the same force, a lighter object accelerates more than a heavier object.

Meaning of One Newton

Using F = ma:

If mass = 1 kg and acceleration = 1 m s⁻²,

F = 1 kg × 1 m s⁻²
F = 1 N

Therefore, one newton is the force that produces an acceleration of 1 m s⁻² in a 1 kg object.

Numerical Example Using F = ma

A 25 kg block is pushed forward with a force of 55 N.

The opposing friction is 50 N.

Net force = 55 N − 50 N
Net force = 5 N

Using F = ma:

a = F/m

a = 5/25

a = 0.2 m s⁻²

The block accelerates at 0.2 m s⁻² in the forward direction.

Weight, Gravitational Force and Momentum

Newton’s second law can also be used to calculate weight.

Momentum describes the quantity of motion possessed by an object.

Weight and Gravitational Force

Weight is the gravitational force exerted by the Earth on an object.

It is calculated using:

F = mg

Where:

• F = weight or gravitational force
• m = mass
• g = acceleration due to gravity

Near the Earth’s surface:

g = 9.8 m s⁻²

For quick estimates, g may be taken as 10 m s⁻².

Example:

Mass of a barbell = 30 kg

Weight = mg

Weight = 30 × 9.8

Weight = 294 N

A weightlifter must apply an upward force of 294 N to hold the barbell steady.

Momentum

Momentum is the product of mass and velocity.

Momentum = Mass × Velocity

p = mv

Where:

• p = momentum
• m = mass
• v = velocity

The direction of momentum is the same as the direction of velocity.

The broader form of Newton’s second law connects force with the rate of change of momentum.

Applications of Newton’s Second Law in Daily Life

The relationship between force and change in velocity explains several safety measures.

Increasing the time taken to stop an object reduces its acceleration and the force of impact.

Catching a Cricket Ball

A fielder moves their hands backwards while catching a fast ball.

This increases the time taken to stop the ball. The acceleration and force acting on the hands become smaller.

Airbags in Vehicles

An airbag inflates during a collision.

It increases the time over which the passenger comes to rest. This reduces acceleration and the force exerted on the body.

Landing Mats

A soft landing mat increases the time taken to stop an athlete after a jump.

This reduces the force exerted on the body.

Breaking a Coconut

A coconut brought down at high speed stops in a very short time after hitting a hard surface.

This rapid change in velocity produces a large force that breaks the shell.

Packing Fragile Objects

Bubble wrap and hay increase the time over which glass objects stop during an impact.

The force becomes smaller, reducing the chance of damage.

Newton’s Third Law of Motion and Action-Reaction Forces

Newton’s third law of motion states:

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

The two forces:

• Are equal in magnitude
• Act in opposite directions
• Occur at the same time
• Act on different objects

They do not cancel each other because they act on separate objects.

Action and Reaction Forces

The following examples show common action and reaction forces:

The words action and reaction do not mean that one force occurs first. Both forces act simultaneously.

Walking and Running

While walking, the foot pushes the ground backwards.

The ground applies a forward frictional force on the foot. This force moves the person forward.

Without enough friction, the foot slips. This is why walking on a wet polished floor is difficult.

Rowing a Canoe

A canoeist pushes water backwards with the paddle.

Water applies an equal and opposite force on the paddle. The canoe moves forward.

A harder backward push produces a larger forward force.

Balloon and Rocket Motion

Air rushing out of a balloon pushes it in the opposite direction.

A rocket works on the same principle.

Its engine expels gases downward. The gases exert an equal upward force on the rocket.

The rocket lifts when the upward force becomes greater than its weight.

Recoil of a Gun

A gun applies a forward force on a bullet.

The bullet applies an equal backward force on the gun. This backward movement is called recoil.

Suppose the force is 2 N.

For a bullet of mass 0.1 kg:

a = F/m
a = 2/0.1
a = 20 m s⁻²

For a gun of mass 5 kg:

a = 2/5
a = 0.4 m s⁻²

The forces are equal, but the lighter bullet has much greater acceleration.

Force and Laws of Motion: Quick Comparison

The chapter on force and laws of motion connects three major ideas.

The first law describes motion without net force. The second explains acceleration, while the third describes interaction between objects.

Forces Acting on a System of Objects

Two or more connected objects may be treated as one system.

This approach makes it easier to calculate their common acceleration.

Suppose two boxes of masses m₁ and m₂ are connected and pulled by an external force F.

Total mass of the system:

Total mass = m₁ + m₂

Acceleration of the system:

a = F/(m₁ + m₂)

The tension in the connecting string is an internal force.

When the complete system is considered, only external forces determine its acceleration.

Class 9 Force and Motion Formulas

The following Class 9 force and motion formulas are useful for numerical questions:

How Forces Affect Motion Notes: Quick Revision

These How Forces Affect Motion notes can be revised through the following key facts:

• Force is a push or pull with magnitude and direction.
• The SI unit of force is the newton.
• Balanced forces produce zero net force.
• Unbalanced forces change velocity.
• Friction acts opposite to motion or attempted motion.
• Newton’s first law explains inertia.
• Newton’s second law gives F = ma.
• Weight is calculated using F = mg.
• Momentum is calculated using p = mv.
• Newton’s third law explains force pairs.
• Action and reaction forces act on different objects.
• Connected objects can be treated as one system.

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