Friction — for Class 8

What Is Friction — and Why Does Your Bicycle Stop?

Push a book across the table. It slows down and stops, even though nothing visible is blocking it. That invisible force doing the stopping is friction — the force that acts between two surfaces in contact, opposing relative motion between them.

Here’s the key insight: friction acts at the surfaces. When two objects touch, their surfaces — no matter how smooth they look — have tiny bumps and irregularities at the microscopic level. These interlocking bumps resist sliding. The rougher the surfaces, the more they interlock, the more friction you get.

Friction always acts opposite to the direction of motion (or intended motion). This is the one rule that never changes. If you push a box to the right, friction on the box acts to the left.

We live with friction every second. Walking works because friction between your shoe and the floor stops your foot from sliding backward. Writing works because friction between the pen and paper allows the ink to deposit. Cars stop because friction between brake pads and wheels converts kinetic energy into heat. Without friction, everything would be an ice rink — permanently.

Key Terms and Definitions

Friction: The force that opposes relative motion or tendency of motion between two surfaces in contact.

Static friction: The friction that acts when an object is at rest and you’re trying to make it move. Static friction can vary from zero up to a maximum value. It exactly matches your applied force — until you exceed its limit.

Sliding friction (kinetic friction): The friction that acts when an object is already moving. It is constant for given surfaces and always less than maximum static friction. This is why it’s harder to start pushing a heavy box than to keep it moving.

Rolling friction: The friction that acts when an object rolls over a surface (like a wheel or a ball). Rolling friction is much smaller than sliding friction — this is exactly why wheels were invented.

Limiting friction: The maximum value of static friction, just before the object begins to move. Once your applied force exceeds limiting friction, the object slides.

Normal force: The force a surface exerts perpendicular to itself on an object resting on it. On a flat surface, this equals the object’s weight. Friction depends directly on this normal force.

Static friction: $f_s \leq \mu_s N$

Sliding friction: $f_k = \mu_k N$

Rolling friction: $f_r = \mu_r N$

Where:

$\mu_s$ = coefficient of static friction
$\mu_k$ = coefficient of kinetic (sliding) friction
$\mu_r$ = coefficient of rolling friction
$N$ = Normal force

Always true: $\mu_s > \mu_k > \mu_r$

The coefficient of friction ( $\mu$ ) is just a number — no units — that tells us how “grippy” two surfaces are together. Rubber on concrete has a high $\mu$ (around 0.8). Ice on ice has a very low $\mu$ (around 0.03). The coefficient depends on the pair of surfaces, not either surface alone.

Types of Friction in Detail

Static Friction — The Stubborn Force

Static friction is self-adjusting. Apply 5 N to a stationary box, static friction becomes 5 N (opposing). Apply 10 N, static friction becomes 10 N. It matches your force until you reach the limiting friction — after that, no more adjusting. The object slides.

Think of static friction as a “guard” that exactly counters your push, up to a limit. Only after you beat the guard does motion start.

Sliding Friction — Constant Once Moving

Once the box slides, friction becomes kinetic/sliding friction. It stays roughly constant regardless of speed (at Class 8 level). This is always less than limiting static friction, which is why objects are easier to keep sliding than to start sliding.

Rolling Friction — Why Wheels Changed Civilization

When a wheel rolls, only a tiny area contacts the ground at any moment, and instead of surfaces scraping across each other, they momentarily push against each other and separate. This gives rolling friction values 10–100 times smaller than sliding friction.

CBSE Class 8 loves asking: “Why do wheels reduce friction?” The answer: rolling friction is much less than sliding friction. Ball bearings in machinery use the same principle.

Factors Affecting Friction

Factor 1 — Nature of surfaces: Rougher surfaces → more friction. Smooth, polished surfaces → less friction. Sandpaper on wood gives more friction than ice on glass.

Factor 2 — Normal force (weight of object): Heavier object → more friction. Press two surfaces harder together and their bumps interlock more. This is why brakes work harder when you press the pedal more firmly.

Factor 3 — NOT the area of contact (common surprise): For the same normal force, a small contact area or a large contact area gives the same friction. This surprises most students. A brick lying flat vs. standing on its edge has the same friction — same weight, same surfaces.

Factor 4 — NOT the speed of sliding: At Class 8 level, sliding friction doesn’t change with speed (for the same pair of surfaces).

Many students write: “More area of contact → more friction.” This is wrong for Class 8 physics. Friction depends on normal force and surface nature — not contact area. Don’t lose marks on this.

Friction: Necessary Evil or Useful Friend?

Where Friction Helps Us

Walking: Without friction between shoes and ground, every step would send you sliding backward.
Brakes: Friction between brake pads and wheels stops vehicles. More friction = better braking.
Writing: Friction between pen/pencil and paper deposits ink/graphite.
Matchstick: Matchhead ignites due to friction with the rough matchbox surface.
Holding objects: Friction between your fingers and a glass lets you grip it.
Knots and nails: Friction in knots holds rope together; friction between nail and wood holds the nail.

Where Friction Hurts Us

Machine wear: Friction between moving parts in engines causes wear and heat generation, reducing efficiency.
Energy loss: Friction converts useful kinetic energy into heat — wasted energy.
Vehicle resistance: Friction in tyres and air resistance slow vehicles, increasing fuel consumption.

How We Reduce Friction

Lubricants (oil, grease, graphite): Fill tiny gaps between surfaces, replacing solid-solid contact with liquid-solid contact. Liquid layers slide past each other with much less friction.
Ball bearings and rollers: Convert sliding friction to rolling friction.
Polishing surfaces: Reduces microscopic roughness.
Air cushions: Hovercraft and air hockey tables use a thin layer of air between surfaces — nearly zero friction.

How We Increase Friction

Grooves and treads: Tyres have treads to grip wet roads. Soles of shoes have patterns.
Rough surfaces: Sandpaper, chalk on a blackboard, grip tape on stairs.
Spikes and studs: Cricketers and footballers wear spiked shoes for grip.

Solved Examples

Example 1 — Easy (CBSE Level)

A girl pushes a box with 20 N of force but the box doesn’t move. What is the friction force on the box?

Since the box is stationary, static friction exactly balances the applied force.

Friction force = 20 N (in the direction opposite to push)

The box is in equilibrium: Net force = 20 N (push) − 20 N (friction) = 0.

Answer: 20 N

Example 2 — Easy (CBSE Level)

A ball bearing is placed between a wheel’s axle and its housing. Why does this reduce friction?

The ball bearing converts sliding friction between the axle and housing into rolling friction between the balls and the surfaces. Since rolling friction is much less than sliding friction, the wheel turns more easily with less energy loss.

Example 3 — Medium (CBSE Level)

A box of weight 100 N is on a rough floor. The coefficient of sliding friction is 0.4. What force is needed to keep it sliding at constant velocity?

At constant velocity, net force = 0. So the applied force must exactly equal the sliding friction force.

f_k = \mu_k \times N

Here $N$ = weight = 100 N (on a flat surface).

f_k = 0.4 \times 100 = 40 \text{ N}

Answer: 40 N must be applied to maintain constant sliding.

“Constant velocity” always means zero net force at Class 8/9 level. So applied force = friction force. Don’t overthink it.

Example 4 — Medium (CBSE + Conceptual)

Why does a car skid on a wet road more easily than on a dry road?

Water acts as a lubricant between the tyre and road surface. It fills the microscopic gaps between the rough tyre tread and road surface, reducing interlocking of bumps. This lowers the coefficient of friction. The smaller friction force is now insufficient to provide the centripetal force needed for turning — the car skids outward.

This is why tyre treads are designed to channel water away from the contact patch, maintaining dry-surface grip even in rain.

Example 5 — Hard (NCERT Application)

A driver applies brakes when the car is moving at 72 km/h. The coefficient of sliding friction between tyres and road is 0.5. Calculate the stopping distance. (Take g = 10 m/s²)

Convert speed: $72 \text{ km/h} = 72 \times \frac{1000}{3600} = 20 \text{ m/s}$

The friction force provides deceleration:

f = \mu_k \times N = \mu_k \times mg

Deceleration: $a = \mu_k \times g = 0.5 \times 10 = 5 \text{ m/s}^2$

Using $v^2 = u^2 - 2as$ (final velocity $v = 0$ ):

0 = 20^2 - 2 \times 5 \times s

s = \frac{400}{10} = 40 \text{ m}

Answer: 40 m stopping distance

At 72 km/h, the car needs 40 m to stop. At higher speeds, stopping distance grows with the square of speed — doubling speed quadruples stopping distance. This is why speed limits exist.

Exam-Specific Tips

CBSE Class 8 Marking Scheme:

Definitions of static, sliding, rolling friction are 1-mark questions — memorise them exactly.
“State the factors affecting friction” is a 2-3 mark question — always list: (1) nature of surfaces, (2) normal force/weight. Specifically mention what does NOT affect friction: contact area, speed.
The comparison $f_{rolling} < f_{sliding} < f_{static\;(limiting)}$ is asked frequently — know the order.
Diagram questions often show a box on a surface with arrows — practice labelling friction direction correctly (always opposing intended motion).

CBSE Board PYQ pattern: Questions on friction appear in Section B (2 marks) and Section C (3 marks) of the Science paper. Numerical questions with coefficient of friction are less common at Class 8 but appear in worksheets and school exams.

NTSE pattern: Conceptual questions like “why is it harder to start sliding than to keep sliding” or “why do vehicles skid more on wet roads” — the reasoning must be sharp and precise.

For Class 8 students targeting JEE later: The foundation built here — especially understanding that friction is a reaction force (static friction self-adjusts) — is the seed of Chapter 5 in Class 11. Get the concepts right now.

Common Mistakes to Avoid

Mistake 1 — Direction of friction on a stationary object: Students write friction is zero if an object isn’t moving. Wrong. If a force is applied to a stationary object that doesn’t move, static friction is equal and opposite to the applied force. Friction is zero only when no force tries to cause motion.

Mistake 2 — Area of contact increases friction: This is the most common wrong answer in Class 8. Contact area does NOT affect friction (for same normal force and surfaces). A brick flat vs. upright has the same friction force.

Mistake 3 — Confusing the order: Students sometimes write sliding friction > static friction (limiting). The correct order is: limiting static friction > sliding friction > rolling friction. Sliding is always less than limiting static — that’s why objects are easier to keep moving than to start.

Mistake 4 — Friction always opposes motion: True, but be precise — friction opposes relative motion or tendency of motion. When you walk forward, friction on your foot from the ground actually acts forward (it stops your foot from sliding backward relative to the ground). This confuses students who think friction only “opposes” in a simple direction.

Mistake 5 — Friction in space: “There’s no friction in space.” Partially true — no air resistance, and objects float so there’s no normal force, hence no surface friction. But if two objects in space are pressed together, friction exists between them. Friction requires contact forces, not gravity specifically.

Practice Questions

Q1. Name the three types of friction and give one example of each.

Sliding (kinetic) friction: A box sliding across a floor. Static friction: A book resting on a tilted surface without sliding. Rolling friction: A ball rolling on a cricket pitch.

Q2. Which is easier to move — a heavy box on a rough floor or on a smooth floor? Explain why.

The heavy box is easier to move on a smooth floor. Friction depends on the nature of surfaces and normal force. Same weight (normal force), but smooth floor has a lower coefficient of friction, so friction force is smaller. Less force needed to move it.

Q3. A car tyre has treads (grooves). A racing car tyre is completely smooth (slick). Which provides better grip in dry conditions and which in wet? Explain.

In dry conditions: Slick tyres provide more grip because the entire tyre surface contacts the road — more interlocking of surfaces, higher friction. This is why F1 cars use slicks in dry races.

In wet conditions: Treaded tyres win. The grooves channel water away from the contact patch, maintaining dry-surface grip. Slick tyres hydroplane (ride on water film) in wet conditions because water can’t escape — friction drops dramatically.

Q4. The coefficient of sliding friction between a wooden block (weight 50 N) and the floor is 0.3. What force is needed to slide it at constant velocity?

At constant velocity, applied force = friction force.

$f_k = \mu_k \times N = 0.3 \times 50 = 15 \text{ N}$

Answer: 15 N

Q5. Why are the handles of tools like knives and scissors made rough or have grooves, while the blades are kept smooth?

The handle needs high friction — rough surfaces increase static friction between hand and handle, ensuring a firm grip without slipping. The blade needs low friction — smooth surfaces reduce friction as the blade cuts through material, making cutting easier and preventing wear.

Q6. A man pushes a 80 kg box but it doesn’t move. If the coefficient of limiting static friction is 0.4, what is the maximum force the man is applying just before the box starts moving? (g = 10 m/s²)

Normal force $N = mg = 80 \times 10 = 800 \text{ N}$

Limiting static friction $= \mu_s \times N = 0.4 \times 800 = 320 \text{ N}$

The man must be applying just under 320 N (at the moment just before sliding). The box moves the instant the applied force exceeds 320 N.

Answer: 320 N

Q7. Why is it difficult to walk on an oily floor?

Oil acts as a lubricant between shoe sole and floor. It fills the microscopic gaps between the surfaces, replacing solid-solid friction with liquid-layer friction. The coefficient of friction drops sharply. The friction force becomes insufficient to stop your foot from sliding when you push backward — so your foot slides, causing you to slip.

Q8. A heavy roller is used to make fields smooth for cricket. Does rolling friction increase or decrease as the field gets smoother? Explain.

Rolling friction decreases as the field gets smoother. Smoother surfaces have fewer microscopic bumps, so the interlocking between roller surface and ground is reduced. Lower roughness → lower coefficient of rolling friction → smaller friction force on the roller. This is why a smooth outfield is easier to run across — less friction slows the ball less too.

FAQs

Why is static friction greater than sliding friction?

When an object is at rest, the surface bumps of both surfaces settle deeper into each other — more interlocking. To start motion, you must break all these interlocking contacts simultaneously. Once sliding begins, the surfaces skim over each other and only some bumps engage at any instant — fewer contacts, less resistance. That’s why limiting static friction > sliding friction.

Does friction depend on speed?

At Class 8 level: No. Sliding friction stays roughly constant regardless of how fast the object moves. At higher physics levels, there are speed-dependent friction effects (viscous drag), but for solid-solid friction at normal speeds, we treat it as constant.

Is friction always bad for machines?

No. Friction is essential in many machine parts — belt drives work because of friction between belt and pulley. Brake systems use friction. Without any friction, nuts and bolts would unscrew themselves, and conveyor belts would slip. Engineers manage friction: increase it where grip is needed, reduce it where energy loss is to be minimized.

Why do bicycle tyres go flat when you brake hard?

Hard braking can cause wheels to lock up and stop rotating. Now instead of rolling friction (tyre rolling on road), you get sliding friction (tyre scraping on road). Sliding friction is much higher than rolling friction — this causes rapid tyre wear and the “skidding” mark on the road. Anti-lock Braking Systems (ABS) in modern vehicles prevent wheel lockup precisely for this reason.

Why do we sprinkle sand on icy roads in winter?

Ice has a very low coefficient of friction — vehicles and pedestrians slide easily. Sprinkling sand increases the roughness of the surface (sand particles embed in the ice and create interlocking bumps), raising the effective coefficient of friction between tyres/shoes and the road surface.

Does the weight of the object affect friction?

Yes — more weight means more normal force, which means more friction. This is why it’s harder to slide a fully loaded box than an empty one across the same floor with the same surface.

Why do climbers use chalk on their hands?

Sweat makes hands smooth and slippery — it acts like a lubricant, reducing friction between hand and rock. Chalk (magnesium carbonate) absorbs the sweat and increases surface roughness, restoring high static friction. Same reason gymnasts and weightlifters use chalk.

Can friction ever act in the direction of motion?

Yes — and this is a beautiful concept. When you walk, the ground’s friction on your foot acts forward (in your direction of motion). Your foot pushes backward against the ground; the ground’s friction reaction pushes your body forward. Friction here is what causes your forward motion. Similarly, when a car accelerates, friction between the driven tyre and road pushes the car forward.