Glossary of Important Terms
- Friction– A contact force that opposes or tends to oppose relative motion between two surfaces in contact. It acts along the tangential direction of the surfaces.
- Static Friction– The force of friction that acts between two surfaces when there is no relative motion between them. It balances the applied force until motion begins.
- Sliding (Kinetic) Friction– The force of friction that acts between two surfaces when one is sliding over the other. It is slightly less than static friction.
- Rolling Friction– The force of friction that comes into play when one object rolls over another. It is much smaller than sliding friction.
- Limiting Friction– The maximum value of static friction just before an object starts moving.
- Lubrication– The process of applying a substance (like oil, grease, or powder) between moving surfaces to reduce friction.
- Lubricant– A substance (solid, liquid, or semi-liquid) used to reduce friction between surfaces (e.g., oil, grease, talcum powder).
- Fluid Friction (Drag)– The frictional force exerted by fluids (liquids and gases) on objects moving through them (e.g., air resistance or water resistance).
- Streamlined Body– A specially designed shape (like that of birds, fish, or aeroplanes) that reduces fluid friction by allowing smooth (streamline) flow of the fluid around it.
- Ball Bearings– Small metal balls placed between moving parts of machines to reduce friction by converting sliding friction into rolling friction.
- Interlocking of Surfaces– The microscopic roughness (irregularities) on surfaces that get locked together, causing friction when one surface moves over another.
- Wear and Tear– The gradual damage to surfaces due to continuous friction, leading to erosion or weakening of materials.
- Self-Adjusting Force (Static Friction)– The property of static friction to increase or decrease in response to the applied force until it reaches its limiting value.
- Hovercraft– A vehicle that moves on a cushion of air, significantly reducing friction on both land and water.
- Adhesive Forces– The intermolecular forces between different surfaces that contribute to friction by forming small “joints” at contact points.
- Treads (on Tyres/Shoes)– Grooves or patterns on surfaces (like tyres or shoe soles) to increase friction and improve grip.
- Air Cushion– A layer of compressed air used to reduce friction between surfaces (e.g., in hovercrafts or some machines).
- Energy Dissipation– The loss of useful energy (as heat) due to friction between moving parts.
Something To Know
A. Fill in the blanks:
- Friction always ____________ any relative motion between the two surfaces that are in contact with each other.
Answer – opposes
- Static friction is always ____________ than sliding friction.
Answer – greater
- Sportsmen use shoes, fitted with spikes, to ____________ friction between their shoes and the ground.
Answer – reduce
- Sprinkling of talcum powder, on the carrom-board, helps to ____________ friction.
Answer – increase
- Frictional force, on an object moving in a fluid, depends on its ____________.
Answer – shape
- The shape of an aeroplane is ____________ to reduce, the effects of friction, due to air.
Answer – streamlined
B. State True or False for the following statements.
- When a body slides over smooth and wet surfaces, the amount of interlocking of irregularities, of the two surfaces in contact, increases.
Answer – False (Smooth and wet surfaces reduce interlocking, decreasing friction.)
- If a car moves eastwards, the force of friction acts southwards.
Answer – False (Friction opposes motion, so it acts westwards, not southwards.)
- Friction depends on how hard the two surfaces press against each other.
Answer – True (Friction increases with greater normal force pressing the surfaces together.)
- The sole of shoes and the tyres of the vehicles are ‘treaded’ to decrease the effects of friction.
Answer – False (Treads increase friction to improve grip, not decrease it.)
- Lubricants can be liquids, semi-liquids or solids.
Answer – True (Examples: oil (liquid), grease (semi-liquid), graphite powder (solid).)
- Friction can be increased by using ball bearings between the moving parts of machines.
Answer – False (Ball bearings reduce friction by converting sliding friction into rolling friction.)
- Fluid friction can be minimised by giving suitable shapes to the objects moving through the fluid.
Answer – True (Streamlined shapes reduce air/water resistance.)
C. Tick (✓) the correct option.
- Suppose your writing desk is tilted to a position where a book kept on it just starts sliding down. The figure, showing the correct direction of frictional force acting on it, is—
Answer – D
- Two boys are applying oppositely directed, and equal in magnitude, forces on a box as shown in the figure.
In such a case, the force of friction, that would exist between the lower end of the box and the ground, would be called—
sliding friction
fluid friction
static friction
rolling friction
Answer – static friction
- Four students were asked to arrange the (frictional) forces, due to rolling, static and sliding friction, in an increasing order. Their arrangements are listed below. The correct arrangement is —
rolling, static, sliding
static, sliding, rolling
rolling, sliding, static
static, rolling, sliding
Answer – rolling, sliding, static
- The energy, ‘used up’ in overcoming friction, gets converted mainly into
sound energy
light energy
heat energy
chemical energy
Answer – heat energy
- Rahul rolls a ball on a wooden surface. The ball covers a certain distance before coming to rest. To make the same ball cover a (much) longer distance, before coming to rest, Rahul should—
spread a newspaper on the wooden surface.
spread a towel on the wooden surface.
sprinkle talcum powder on the wooden surface.
spread a jute bag on the wooden surface.
Answer – sprinkle talcum powder on the wooden surface.
- A block is sliding on a horizontal surface. The force of friction between the two can be increased by—
decreasing the area of contact of the block with the surface.
applying a layer of some lubricant on the surface.
by polishing the surface.
by putting a second identical block on top of the given block.
Answer – by putting a second identical block on top of the given block.
D. Answer the following questions in brief:
- Define the terms (a) static friction (b) sliding friction.
Answer – (a) Static friction – The frictional force that acts between surfaces when they are at rest relative to each other, preventing motion until a certain force is applied.
(b) Sliding friction – The frictional force that opposes the relative motion between two surfaces when one is sliding over the other.
- State the meanings of the terms (a) rolling friction (b) fluid friction.
Answer – Meanings:
(a) Rolling friction – The frictional force that occurs when an object rolls over a surface (e.g., wheels on the ground). It is much smaller than sliding friction.
(b) Fluid friction – The resistance experienced by an object moving through a fluid (liquid or gas), such as air resistance or water drag.
- State the likely cause of friction.
Answer –Cause of friction:
Friction is caused by the interlocking of microscopic irregularities (roughness) between two surfaces in contact. When one surface moves or tries to move over another, these irregularities resist the motion.
- A box is resting on the floor. To move it, a (variable) force is applied as shown in the diagram below.
(a) Give the (likely) value of the force of limiting friction.
Answer – More than 2f and less than 3f.
(Assuming the diagram shows increasing force until the box moves.)
(a) Limiting friction ≈ F₃ (the maximum static friction just before the box starts moving).
(b) Why does the box stay at rest in the cases corresponding to Fig (a) and Fig (b)?
Answer – It is because the force applied is less than that of the limiting friction as in figure (a) and figure (b).
(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.
Answer – The direction of the frictional force is always opposite to the motion of box in given figure.
The kind of frictional force that comes into play, in
Case a-Static Friction
Case b-Static Friction
Case c-Sliding Friction
- Name the kind of frictional friction that comes into play, when a book, kept on a collection of cylindrical pencils, is moved by pushing it.
Answer – Friction type for book on pencils:
Rolling friction (because the cylindrical pencils roll as the book moves, reducing friction compared to sliding).
- Why are objects given special shapes when they are moving through fluids? Write the name given to these special shapes?
Answer – Special shapes in fluids:
Objects are given streamlined shapes (tapered at the ends) to:
Minimize fluid friction (drag).
Allow smooth airflow/waterflow around the object, reducing resistance.
Examples: Aeroplanes, fish, and submarines have streamlined designs.
E. Answer the following questions.
- Define Force of friction. List the factors affecting the force of friction. Explain with examples.
Answer – Force of Friction: Definition, Factors, and Examples
Definition:
Friction is a contact force that opposes the relative motion (or tendency of motion) between two surfaces in contact. It acts parallel to the surfaces and resists movement.
Factors Affecting Friction:
Nature of Surfaces – Rougher surfaces produce more friction (e.g., rubber on concrete) than smooth surfaces (e.g., ice on metal).
Normal Force (Weight) – More weight pressing surfaces together increases friction (e.g., pushing a heavy box vs. a light one).
Area of Contact (Independent in most cases) – Friction does not depend on the area of contact (e.g., wide vs. narrow tyres have the same friction if weight is equal).
Examples:
High Friction: Car tyres on a road (for grip).
Low Friction: Ice skates on ice (to glide smoothly).
- A ball is allowed to roll down an inclined wooden plane from a given height. At the foot of the inclined plane, it moves on a horizontal surface, differently ‘covered’, one by one, as shown in the following figures.
In which case, is the ball likely to move the longest distance? Give reason for your answer.
Answer – In case ‘c’ (glass sheet), the ball will move for longest distance it is because,
glass surface is smooth
less interlocking
less friction.
- Explain why sliding friction is slightly less than static friction.
Answer – Static friction must be overcome to start motion, requiring maximum force to ‘unlock’ surface irregularities.
Sliding friction is slightly less because once moving, surfaces do not fully re-interlock, reducing resistance.
Analogy: Pushing a heavy box is hardest at first (static friction), but easier to keep moving (sliding friction).
- Give reasons for the following—
(a) We tend to slip when we step on a banana peel.
Answer – Slipping on a Banana Peel
Banana peel reduces friction by creating a slippery layer between the foot and ground, preventing grip.
(b) It is easier to push a lighter box than a similar heavy box on the same floor.
Answer – Easier to Push a Lighter Box
Friction depends on normal force (weight). A lighter box has less friction opposing motion.
(c) The force, needed to start a cart, is (somewhat) greater than the force needed to keep it moving with a uniform speed.
Answer – Starting Force > Moving Force
Static friction (to start moving) > Sliding friction (to keep moving).
(d) Sportsmen use shoes fitted with spikes.
Answer – Spikes on Sports Shoes
Spikes increase friction by digging into the ground, preventing slips during quick movements.
(e) Ball-bearings are used in machines.
Answer – Ball-Bearings in Machines
They replace sliding friction with rolling friction, reducing energy loss and wear.
(f) Tyres of the vehicles need to be changed regularly.
Answer – Regular Tyre Changes
Treads wear out, reducing grip. Fresh tyres restore friction for safety.
(g) Machines parts are frequently oiled or greased.
Answer – Oiling/Greasing Machine Parts
Lubricants reduce friction, preventing overheating and wear.
(h) Sport cars, aeroplanes and boats are designed to have a streamlined shape.
Answer – Streamlined Shapes (Cars, Aeroplanes, Boats)
Reduces air/water resistance (fluid friction), allowing faster, fuel-efficient movement.
(i) Metal chains are wrapped on tyres when they run on icy roads.
Answer – Chains on Icy Tyres
Chains increase friction by biting into ice, preventing skidding.
(j) Rollers are used for transporting luggage.
Answer – Rollers for Luggage
Rolling friction is much less than sliding friction, making it easier to move heavy loads.
- State and explain, with examples, how friction can help the cause of motion in certain situations.
Answer – Friction is not always an opponent—it enables motion in many cases:
Walking/Running
Friction between shoes and ground pushes us forward (action-reaction).
Without friction, we’d slip (e.g., on ice).
Vehicle Movement
Tyres grip the road due to friction, allowing acceleration and steering.
Smooth tyres on wet roads cause skidding (lack of friction).
Writing with a Pencil
Friction between pencil tip and paper allows marks to form.
Braking in Vehicles
Friction between brake pads and wheels slows down the car.
Conclusion: While friction often resists motion, it is essential for controlled movement in daily life.
Value Based Questions
Ramit and Kush, both students of Class VIII, would often strongly argue with each other about the different aspects of their day-to-day school life. However, they made a very good ‘doubles team’ of their school, for the interschool badminton tournaments. When playing together, they would forget all their differences and play as a ‘team’ to ensure their school’s victory.
Their science teacher, who was also their badminton coach, would quote their example. He would explain to his students how the force of friction can be a ‘good friend’ and ‘a source of help’ even though it was usually viewed only as a ‘trouble maker’.
- State the values displayed by Ramit and Kush.
Answer – Values Displayed by Ramit and Kush
Teamwork & Cooperation: Despite their arguments, they work together harmoniously in badminton.
Sportsmanship: They prioritize their school’s victory over personal differences.
Discipline & Focus: They concentrate on the game, setting aside conflicts.
Respect for Coach/Teacher: They heed their teacher’s advice on friction and teamwork.
- Give two examples of situations in which the force of friction is a ‘source of help’.
Answer – Two Examples Where Friction is a ‘Source of Help’
Walking/Running: Friction between shoes and the ground prevents slipping, allowing us to move forward.
Vehicle Brakes: Friction between brake pads and wheels helps stop moving cars safely.
- Stage a play in which different members of two teams give examples to show that the force of friction is (i) a friend (ii) a foe.
Answer – Play Script: “Friction – Friend or Foe?”
Characters:
Team A (Friction is a Friend)
Team B (Friction is a Foe)
Narrator (Science Teacher/Coach)
Scene: A classroom debate moderated by the teacher.
Narrator: “Today, we’ll debate whether friction is a friend or foe. Team A, present your case!”
Team A (Friend):
“Without friction, we couldn’t walk! Try running on ice—no grip, no motion!”
“Cars need friction to move! Tyres grip the road, else they’d spin uselessly!”
Team B (Foe):
“Friction wears out shoes and tyres, costing money!”
“It heats up machines, wasting energy and causing damage!”
Narrator: “Both teams are right! Friction is a necessary friend—it helps motion but must be controlled. Like Ramit and Kush, balance is key!”
Moral: Friction, like teamwork, has pros and cons. Wise use makes it beneficial!
Something To Do
- Imagine that friction were to suddenly vanish altogether. Write a short story/play on how would our lives be affected.
Answer – Short Story: “A World Without Friction”
Title: The Day Friction Disappeared
Scene 1: Morning Chaos
Rahul wakes up and tries to get out of bed—but slides straight onto the floor. His socks won’t grip the tiles. Outside, cars spin their wheels uselessly on the roads, unable to move. People struggle to walk, slipping like they’re on ice.
Scene 2: School Disaster
At school, pencils slip from fingers, making writing impossible. The basketball game turns into a comedy—players can’t stop or change direction, sliding past the hoop. The canteen lunch spills everywhere as plates won’t stay on tables.
Scene 3: Transportation Collapse
Trains overshoot stations because brakes fail. Airplanes can’t land safely—their wheels skid endlessly on runways. Even zippers and buttons won’t stay closed without friction!
Moral: Friction, though annoying sometimes, is essential for control and movement. Life without it would be chaotic and dangerous!
- List some sports/games where friction is a ‘help’ or ‘a source of trouble’. Discuss your list with your friends with some supporting pictures.
Answer – Sport/Game – Friction as a HELP – Friction as a TROUBLE
Football – Cleats grip grass for quick turns. – Too much friction slows the ball on dry turf.
Ice Hockey – Blades dig into ice for control. -Low friction makes players slip if unbalanced.
Rock Climbing – Rough shoes/hands grip rocks. – Rope friction causes burns if uncontrolled.
Car Racing – Tyres grip the track for speed. – Friction heats tyres, causing wear.
Bowling – Friction stops the ball at the pins. – Oiled lanes reduce friction, making it tricky.
Activity:
Collect pictures of spiked shoes, rough climbing holds, and smooth racing tyres.
Discuss how each sport adjusts friction (e.g., waxing skis for speed, chalking hands in gymnastics).
- In the Activity 1 of this Chapter, what is likely to happen if you change the angle which the tray makes with the table top. Does the angle, made by the inclined plane with respect to horizontal, affect the sliding? Discuss your findings with your teacher. Does the angle (at which sliding just starts) depend on the nature of the two surfaces in contact? Find the answer using different materials/toy cars and so on.
Answer – Activity 1: Investigating Angle & Sliding Friction
Experiment Setup:
Place a tray at an angle on a table.
Slowly increase the angle until an object (e.g., toy car) just starts sliding.
Observations:
Steeper Angle = Faster Sliding
Higher angles reduce the normal force, making gravity overcome friction more easily.
Depends on Surface Material
Rough surfaces (sandpaper): Sliding starts at a higher angle (more friction).
Smooth surfaces (glass): Sliding starts at a lower angle (less friction).
Conclusion:
The critical angle (where sliding begins) varies with surface roughness.
Real-world link: Roads are built with gentle slopes to prevent cars from sliding unintentionally.
Teacher Discussion:
Why do icy roads cause accidents? (Low friction → sliding at small angles).
How do mountain roads use gravel to increase friction?
Extension: Test with different objects (rubber eraser vs. metal block) to see how weight and material affect the angle.
Key Takeaways:
Friction’s Role: Without it, basic tasks become impossible.
Sports Adaptation: Athletes manipulate friction for performance.
Science of Sliding: Angle and surface texture determine when motion starts.
Activity Idea: Host a “Friction Olympics” with games testing grip (e.g., tug-of-war on different surfaces)!
Additional Questions
Short Answer Questions (1-2 marks)
- What is friction?
Answer: Friction is a contact force that opposes the relative motion (or tendency of motion) between two surfaces in contact.
- Why does friction produce heat?
Answer: When surfaces rub against each other, their irregularities interlock and resist motion, converting kinetic energy into heat.
- Why is rolling friction less than sliding friction?
Answer: Rolling involves less surface contact (only a small point touches at a time), reducing interlocking compared to full-surface sliding.
- Why do car tyres have treads?
Answer: Treads increase friction by channelling water away, preventing skidding on wet roads.
- Why do we sprinkle powder on carrom boards?
Answer: Powder acts as a lubricant, reducing friction and allowing smooth movement of the striker.
- Why do athletes wear spiked shoes?
Answer: Spikes dig into the ground, increasing friction for better grip during running/jumping.
- What is the direction of frictional force?
Answer: Friction always acts opposite to the direction of motion (or intended motion).
- Why is it harder to push a heavy box than a light one?
Answer: Heavy boxes have greater normal force, increasing friction between surfaces.
Long Answer Questions (3-5 marks)
- Explain how friction is both a friend and a foe. Give examples.
Answer: Friend:
Enables walking (friction between shoes and ground).
Allows vehicles to move (tyre-road friction).
Foe:
Causes wear and tear (e.g., soles of shoes wearing out).
Wastes energy as heat in machines.
- Describe an experiment to show that friction depends on the nature of surfaces.
Answer: Materials: Wooden block, sandpaper, glass sheet, spring balance.
Steps:
Pull the block on different surfaces (sandpaper/glass).
Measure force using the spring balance.
Observation: More force is needed on rough surfaces (sandpaper) than smooth ones (glass).
Conclusion: Friction increases with surface roughness
- Why are vehicles designed with streamlined shapes? Explain with examples.
Answer: Purpose: To reduce air resistance (fluid friction).
Examples:
Aeroplanes: Tapered noses cut through air.
Speedboats: Narrow fronts minimize water drag.
Result: Saves fuel and increases speed.
- Compare static, sliding, and rolling friction with examples.
Answer:
Type | Definition | Example |
Static | Acts on stationary objects | Pushing a heavy box (before it moves). |
Sliding | Opposes motion of sliding objects | Sliding a book on a table. |
Rolling | Acts when objects roll | Bicycle wheels moving on road. |
Order: Static > Sliding > Rolling friction.
- How does lubrication reduce friction? Give real-life applications.
Answer: Mechanism: Lubricants (oil, grease) form a thin layer between surfaces, preventing direct contact.
Applications:
Engines: Oil reduces wear in car engines.
Doors: Grease stops hinges from squeaking.
Application-Based Questions –
- A car is stuck in muddy soil. What can be done to move it?
Answer: Place wooden planks/sand under tyres to increase friction.
Use 4WD mode to distribute force evenly.
- Why do rockets have a pointed nose?
Answer: To minimize air friction during launch, allowing smoother passage through the atmosphere.
- How do brakes work in bicycles?
Answer: Brake pads press against the wheel rim, creating friction that converts kinetic energy into heat, slowing the bike.
HOTS (Higher Order Thinking Skills) Questions –
- If friction vanishes, how would it affect sports like football or cricket?
Answer: Football: Players couldn’t run or stop; the ball would never slow down.
Cricket: Bowlers couldn’t grip the ball; batsmen would slip while batting.
- Why do astronauts wear special boots on the Moon?
Answer: The Moon’s surface has less gravity and loose dust, so boots are designed with deep treads to increase friction and prevent slipping.
- Why are roads banked on curves?
Answer: Banking uses gravity to counter friction, preventing cars from skidding outward at high speeds.
Diagram-Based Questions.
- Label the forces acting on a box being pushed on a floor.
Answer:
→ Applied force (Push)
← Frictional force
↓ Weight (Gravity)
↑ Normal force
- Draw a streamlined shape and a non-streamlined shape. Which one has less friction?
Answer: Streamlined: Tapered ends (e.g., aeroplane).
Non-streamlined: Flat front (e.g., brick).
Less friction: Streamlined shape.
Practical Activity Questions
- Design an experiment to show that friction generates heat.
Answer: Rub hands together vigorously for 30 seconds.
Observation: Palms feel warm.
Conclusion: Friction converts motion energy into heat.
- How would you test if wider tyres have more friction than narrow ones?
Answer: Use toy cars with different tyre widths on the same track.
Measure distance travelled after rolling from a ramp.
Result: Wider tyres grip better (stop sooner due to higher friction).
Summary Table: Types of Friction
Type | When It Acts | Example |
Static | Before motion starts | Pushing a heavy cupboard. |
Sliding | During sliding motion | Dragging a chair. |
Rolling | During rolling motion | Ball rolling on ground. |
Fluid | In liquids/gases | Swimming in water. |