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

Chapter 6. How Forces Affect Motion

Friction is one of the most common contact forces that we experience every day. It plays an important role in walking, writing, driving vehicles and using machines. Although friction opposes motion, it is both useful and harmful depending on the situation. This section explains friction and its applications with classroom-oriented examples.

29. Friction

Whenever two surfaces are in contact, a force acts between them that opposes their relative motion. This opposing force is called friction.

Definition

Friction is a contact force that opposes the relative motion or the tendency of motion between two surfaces in contact.

Characteristics of Friction

• Contact Force – Friction acts only when two surfaces are in contact.
• Opposes Motion – It always acts opposite to the direction of motion.
• Acts on Both Surfaces – Equal frictional forces act on both objects.
• Produces Heat – Some mechanical energy is converted into heat.

30. Types of Friction

Depending on the condition of motion, friction is classified into different types.

Classification

• Static Friction – Acts on an object at rest and prevents it from moving.
• Sliding Friction – Acts when one surface slides over another.
• Rolling Friction – Acts when an object rolls over a surface.
• Fluid Friction (Drag) – Acts when an object moves through air or water.

31. Factors Affecting Friction

The amount of friction depends on the nature of the surfaces and the force pressing them together.

Main Factors

• Nature of Surfaces – Rough surfaces produce more friction than smooth surfaces.
• Force Between Surfaces – Greater pressure between surfaces increases friction.
• Type of Motion – Rolling friction is usually less than sliding friction.

32. Advantages of Friction

Without friction, many daily activities would become impossible.

Applications

• Walking – Friction between our feet and the ground prevents slipping.
• Writing – Friction allows a pen or pencil to write on paper.
• Driving Vehicles – Tyres grip the road because of friction.
• Braking – Friction helps stop moving vehicles safely.
• Holding Objects – Friction allows us to grip and hold objects firmly.

33. Disadvantages of Friction

• Wear and Tear – Machine parts gradually wear out.
• Heat Generation – Mechanical energy is lost as heat.
• Energy Loss – More fuel or energy is required to overcome friction.
• Reduced Efficiency – Machines become less efficient over time.

34. Methods to Reduce Friction

Method	Purpose
Lubrication	Reduces friction between moving parts.
Ball Bearings	Convert sliding friction into rolling friction.
Polishing	Makes surfaces smoother.
Streamlined Shape	Reduces air and water resistance.

35. Methods to Increase Friction

Method	Example
Making Surfaces Rough	Tread patterns on tyres.
Using Spikes	Sports shoes and mountain climbing shoes.
Applying Sand	Sand spread on slippery roads.
Grooved Brake Pads	Improve braking efficiency.

36. Applications of Force

Force is used in almost every activity of our daily life.

Examples

• Opening or Closing Doors
• Pushing Shopping Trolleys
• Playing Football or Cricket
• Lifting Heavy Objects
• Operating Machines
• Launching Rockets

37. Applications of Newton's Laws

• Seat Belts – Prevent passengers from moving forward suddenly due to inertia.
• Airbags – Increase the time of impact and reduce the force acting on passengers.
• Catching a Cricket Ball – Players move their hands backward to reduce the impact force.
• Heavy Vehicles – Require greater force to accelerate because they have greater mass.
• Rocket Launch – A large force is required to overcome Earth's gravitational pull.

38. ATP Education Classroom Concept Builder

Concept	Key Idea
Force	Push or pull.
Balanced Force	Net Force = 0.
Unbalanced Force	Changes motion.
Friction	Opposes motion.
Newton's First Law	Law of Inertia.
Newton's Second Law	Force is proportional to acceleration.
Weight	Gravitational force acting on a body.

39. Classroom Thinking Questions

• Why is it difficult to walk on a wet floor?
• Why do vehicle tyres have deep grooves?
• Why are ball bearings used in machines?
• Why do athletes wear spiked shoes?
• Why do brakes become hot after continuous use?
• Why is oil applied to bicycle chains?

ATP Education Exam Booster

For CBSE examinations, always identify which force is acting, whether friction is useful or harmful, and how Newton's laws explain the given situation. Questions based on friction, seat belts, airbags, ball bearings, tyre treads and real-life applications are frequently asked in competency-based and case-study assessments.

Class 9, all subjects CBSE Notes in english medium, cbse class 9 Science Exploration notes, class 9 Science Exploration notes english medium, cbse 9 Science Exploration cbse notes, class 9 Science Exploration revision notes, cbse class 9 Science Exploration study material, ncert class 9 science notes pdf, class 9 science exam preparation, cbse class 9 physics chemistry biology notes