Chapter-How Forces Affect Motion Science Exploration class 9 in english Medium CBSE Notes

Chapter 6. How Forces Affect Motion

This section explains Newton's laws of motion, inertia, the relationship between force, mass and acceleration, and the concept of weight. These principles help us understand why objects start moving, stop moving and change their speed in everyday life.

19. 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.

Definition

An object does not change its state of rest or uniform motion unless a net external force acts on it.

Key Points

• Objects at Rest – Remain at rest until pushed or pulled.
• Moving Objects – Continue moving with constant velocity if no unbalanced force acts.
• Balanced Forces – Do not change the state of motion.
• Unbalanced Forces – Change the state of motion.

Examples

• A book remains on a table until someone pushes it.
• A football remains stationary until it is kicked.
• A moving bicycle slows down because friction acts as an external force.

20. Inertia

Every object has a natural tendency to resist any change in its state of rest or motion. This property is known as inertia.

Definition

Inertia is the property of a body by which it resists any change in its state of rest or uniform motion.

Factors Affecting Inertia

• Mass – Greater the mass, greater the inertia.
• State of Motion – Inertia acts whether the object is at rest or moving.

21. Types of Inertia

Inertia of Rest

The tendency of an object at rest to remain at rest.

Example

• Passengers move backward when a bus starts suddenly.

Inertia of Motion

The tendency of a moving object to continue moving.

Example

• Passengers move forward when a moving bus stops suddenly.

Inertia of Direction

The tendency of an object to resist a change in its direction of motion.

Example

• Passengers are pushed sideways when a vehicle takes a sharp turn.

22. Newton's Second Law of Motion

Newton's Second Law explains how force, mass and acceleration are related.

Definition

The acceleration produced in an object is directly proportional to the net force acting on it and inversely proportional to its mass.

Important Relationships

• Greater Force → Greater acceleration.
• Greater Mass → Smaller acceleration for the same force.
• Acceleration → Always in the direction of the net force.

23. Mathematical Expression of Force

Newton's Second Law is represented mathematically by the following equation.

Formula

F = m × a

Symbol	Meaning	SI Unit
F	Force	newton (N)
m	Mass	kilogram (kg)
a	Acceleration	m/s²

24. One Newton

Definition

One newton is the force required to produce an acceleration of 1 m/s² in an object of mass 1 kg.

Formula

1 N = 1 kg × 1 m/s²

25. Weight of an Object

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

Formula

Weight (W) = m × g

Where

• m → Mass of the object (kg)
• g → Acceleration due to gravity (≈ 9.8 m/s²)

26. Difference Between Mass and Weight

Mass	Weight
Amount of matter in an object.	Gravitational force acting on an object.
SI Unit: kilogram (kg)	SI Unit: newton (N)
Remains constant everywhere.	Changes with gravity.
Scalar quantity.	Vector quantity.

27. Applications of Newton's Second Law

• Airbags in Cars – Increase the time of impact and reduce the force on passengers.
• Catching a Cricket Ball – Moving the hands backward reduces the impact force.
• Seat Belts – Protect passengers during sudden stops.
• Sports Equipment – Helmets and pads reduce the force of impact.

Newton's Third Law of Motion

Newton's Third Law of Motion states that whenever one object exerts a force on another object, the second object simultaneously exerts an equal force in the opposite direction on the first object. These two forces are called an action–reaction pair.

Definition

For every action, there is an equal and opposite reaction.

Characteristics of Newton's Third Law

• Equal Magnitude – The action and reaction forces are always equal in magnitude.
• Opposite Direction – The two forces always act in opposite directions.
• Act on Different Objects – Action and reaction never act on the same object.
• Simultaneous Forces – Both forces occur at the same instant.

Examples

• Walking – We push the ground backward, and the ground pushes us forward.
• Swimming – A swimmer pushes water backward, and the water pushes the swimmer forward.
• Rocket Launch – Hot gases are expelled downward, and the rocket moves upward.
• Jumping from a Boat – The person moves forward while the boat moves backward.

Momentum

Momentum is the quantity of motion possessed by a moving object. It depends on both the mass of the object and its velocity. An object with greater mass or higher velocity has greater momentum.

Definition

Momentum is the product of the mass of an object and its velocity.

Formula

Momentum (p) = Mass (m) × Velocity (v)

SI Unit

The SI unit of momentum is kilogram metre per second (kg·m/s).

Factors Affecting Momentum

• Mass – Greater the mass, greater the momentum.
• Velocity – Greater the velocity, greater the momentum.

Characteristics of Momentum

• Vector Quantity – Momentum has both magnitude and direction.
• Depends on Motion – A body at rest has zero momentum.
• Direction – Momentum acts in the same direction as velocity.

Examples

• Fast Cricket Ball – A fast-moving cricket ball has greater momentum than a slowly moving ball.
• Running Athlete – An athlete gains more momentum by increasing speed.
• Moving Truck – A loaded truck has greater momentum than a motorcycle moving at the same speed because its mass is larger.
• Running Football Player – A player running at high speed has greater momentum than when walking.

Applications of Momentum

• Road Safety – Heavy vehicles possess greater momentum and require a longer distance to stop.
• Sports – Momentum helps explain batting, bowling and kicking actions.
• Transportation – Engineers consider momentum while designing braking systems.
• Rocket Science – The motion of rockets is closely related to the concept of momentum.

​Relationship Between Momentum and Force

A greater force changes the momentum of an object more rapidly. Therefore, force is directly related to the change in momentum.

ATP Education Tip

Remember these key concepts: Newton's Third Law explains action and reaction forces, while momentum explains the quantity of motion of an object. Both concepts help explain real-life situations such as walking, swimming, rocket launches, collisions and sports.

28. ATP Education Concept Builder

• Newton's First Law → Explains inertia.
• Inertia → Resistance to change in the state of motion.
• Greater Mass → Greater inertia.
• Newton's Second Law → F = ma.
• Force → Directly proportional to acceleration.
• Weight → W = mg.
• Mass → Constant everywhere.
• Weight → Depends on gravity.

ATP Education Exam Booster

Remember these four important concepts for CBSE examinations: Newton's First Law explains inertia, Newton's Second Law gives the relationship F = ma, one newton is the SI unit of force, and weight is calculated using W = mg. Questions based on inertia, mass versus weight, and force calculations are frequently asked in objective, competency-based and numerical sections.

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