⚛️ Physics · Topic 8 of 10
Start with the study notes on Friction (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Friction is a force that opposes the motion, or attempted motion, of one surface over another. It always acts opposite to the direction of motion.
Examples:
Even surfaces that look smooth have tiny bumps and hollows. When two surfaces touch, these irregularities catch on each other and resist sliding. This resistance is friction.
Friction depends mainly on:
Example: A heavy box is harder to push than a light one, because it presses harder against the floor.
Remember: Friction is a contact force: it acts only between surfaces that are touching.
There are three main types of friction between solid surfaces.
Acts when an object is at rest and a force tries to move it.
Example: You push a heavy cupboard, but it does not move at first. Static friction is holding it in place.
Acts when one surface slides over another.
Example: A book pushed across a table slides for a while, then stops.
Acts when an object rolls over a surface.
Example: A ball rolling on the ground slowly comes to rest.
For the same object and surface:
Static friction > Sliding friction > Rolling friction
This explains why:
Liquids and gases also resist objects moving through them. This is called fluid friction, or drag. It increases as the object moves faster.
Examples:
Streamlining means giving an object a smooth, pointed shape so that it moves through air or water with less drag.
Example: Fish, birds, cars, boats and aeroplanes all have streamlined shapes.
Friction is called a necessary evil because it is both useful and harmful.
We increase friction when we need a better grip or want to stop slipping.
We reduce friction when we want things to move easily or machines to run smoothly.
When two surfaces rub together, some energy is turned into heat.
Examples:
| Misconception | Correct understanding |
|---|---|
| Friction is always harmful. | Friction is essential for walking, writing and braking. |
| Smooth surfaces have no friction. | Every real surface has some friction, even if it looks smooth. |
| Rolling friction is greater than sliding friction. | Rolling friction is much smaller, which is why wheels help. |
| The colour of a surface affects friction. | What matters is the material, how rough it is and how hard the surfaces press together. |
| Friction acts only on moving objects. | Static friction acts even on an object at rest that you are trying to move. |
Why does a rolling ball eventually stop?
Rolling friction with the ground, and air resistance, oppose its motion.
Why is it hard to start moving a heavy cupboard?
Static friction, the largest kind, must be overcome first.
Name the three types of friction between solid surfaces.
Static, sliding and rolling friction.
Why do tyres have treads?
To increase friction and grip on the road.
How do ball bearings reduce friction?
They change sliding friction into much smaller rolling friction.
Why are fish and aeroplanes streamlined?
Their shape reduces drag in water or air.
Give two examples of friction producing heat.
Rubbing your hands together, and striking a match.
Why is friction called a necessary evil?
We need it for walking, writing and braking, but it also wastes energy and wears things out.
Key takeaway: Friction can be useful or harmful. We increase it when we need grip, and reduce it when we need smooth movement.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
The force that opposes the relative motion between two surfaces in contact is called:
Answer: A — Friction
Friction is the force that acts between two surfaces in contact and opposes their relative motion. It always acts in the direction opposite to the motion.
Q54. easy
Friction acts between:
Answer: A — Two surfaces in contact
Friction acts between two surfaces that are in contact and opposes their relative motion. It is a contact force.
Q2. medium
Friction always acts in a direction ______ to the motion of an object.
Answer: A — Opposite
Friction acts opposite to the direction in which an object moves (or tends to move), which is why it slows down or stops moving objects.
Q3. medium
Friction is caused mainly by:
Answer: A — The irregularities (roughness) on the surfaces in contact interlocking
Even surfaces that look smooth have tiny bumps and hollows (irregularities). When two surfaces touch, these irregularities interlock, and friction is the resistance to sliding them past each other.
Q6. medium
It is harder to walk on a smooth WET floor or ice than on dry rough ground because:
Answer: A — There is very little friction on smooth/wet surfaces
Smooth wet floors and ice have very little friction, so our feet slip and we cannot grip. Friction is needed to walk - it grips our feet to the ground and stops us slipping.
Q10. medium
The friction that acts when an object ROLLS over a surface is called:
Answer: A — Rolling friction
Rolling friction acts when a wheel or ball rolls over a surface. Rolling friction is much smaller than sliding friction, which is why wheels make moving loads so much easier.
Q12. medium
The invention of the WHEEL greatly reduced the effort of moving loads because rolling friction is:
Answer: A — Much less than sliding friction
A wheel converts sliding into rolling. Since rolling friction is far smaller than sliding friction, wheels let us move heavy loads with much less force - one of humanity's most important inventions.
Q13. medium
Which of these is a way to REDUCE friction?
Answer: A — Applying oil or grease (a lubricant) between surfaces
Lubricants like oil and grease form a thin layer between surfaces, filling the irregularities and letting them slide easily. This reduces friction and wear in machines.
Q16. medium
Which of these INCREASES friction (a useful thing in some cases)?
Answer: A — Making surfaces rougher, like the grooves (treads) on tyres and shoe soles
Rough surfaces increase friction. Tyres and shoe soles have grooves/treads to increase grip (friction) with the road/ground, preventing slipping - here more friction is helpful.
Q18. medium
Friction produces:
Answer: A — Heat (and wears out surfaces)
Friction produces heat - this is why rubbing our hands together warms them, and why machine parts get hot. Friction also causes wear and tear, gradually damaging the rubbing surfaces.
Q20. medium
Friction is described as a 'necessary evil' because it:
Answer: A — Is useful in many ways (walking, gripping) but also causes wear and wastes energy
Friction is a 'necessary evil': we could not walk, write, hold objects or stop vehicles without it, but it also wears out machine parts, produces unwanted heat, and wastes energy. We increase it or reduce it as needed.
Q22. medium
Which of these could NOT happen without friction?
Answer: A — Writing with a pencil, gripping objects, and stopping a moving vehicle
Friction is needed for writing (pencil on paper), holding objects (grip), and braking (stopping vehicles). Without friction, pencils would not leave marks, objects would slip from our hands, and brakes would not work.
Q24. medium
Sportspersons wear shoes with SPIKES or studs to:
Answer: A — Increase friction (grip) with the ground so they do not slip
Spikes and studs dig into the ground, greatly increasing grip (friction). This stops athletes and footballers from slipping while running, turning or kicking on grass or tracks.
Q26. medium
Streamlining (shaping objects to be pointed/smooth) is used in aeroplanes, cars and fish to:
Answer: A — Reduce fluid friction (drag) so they move easily through air or water
A streamlined (tapered, smooth) shape lets air or water flow past with less resistance, reducing fluid friction (drag). This is why cars, planes, boats, birds and fish have streamlined shapes to move faster and save energy.
Q28. medium
Fish, birds and aeroplanes have streamlined shapes to REDUCE:
Answer: A — Fluid friction (drag)
Streamlined shapes cut down fluid friction (drag) as objects move through water or air, allowing fish to swim, birds to fly and aeroplanes to travel with less resistance and energy.
Q30. medium
Sprinkling talcum powder on a carrom board is done to:
Answer: A — Reduce friction so the coins/striker slide easily and far
Fine powder acts like tiny ball bearings, reducing friction between the coins and the board so they slide smoothly and travel farther. Here, reducing friction is useful.
Q32. medium
Which surface would produce the MOST friction for a sliding object?
Answer: A — A rough concrete floor
A rough concrete floor has many irregularities, producing the most friction. Polished marble, glass and oily surfaces are smooth or slippery, giving much less friction.
Q34. medium
Which factor does NOT affect the amount of friction between two surfaces?
Answer: A — The colour of the surfaces
Friction depends on the roughness of the surfaces and how hard they are pressed together (and the nature of the materials). The colour of the surfaces has no effect on friction.
Q36. medium
Which of these would you want to have HIGH friction?
Answer: A — The sole of a climbing shoe (for grip)
A climbing shoe needs high friction (grip) so the climber does not slip. Machine parts, skis and boats need LOW friction to move easily - so friction is increased or reduced depending on the purpose.
Q38. medium
Which statement about friction and surfaces is TRUE?
Answer: A — Rougher surfaces have more friction; smoother surfaces have less friction
The rougher the surfaces, the more their irregularities interlock, so the greater the friction. Smoothing surfaces (or lubricating them) reduces friction.
Q40. medium
Why is it difficult to hold a wet, soapy object?
Answer: A — Soap and water reduce friction, so the object slips out of the hand
Soap and water act as lubricants, greatly reducing the friction between the object and your hand. With so little grip (friction), the slippery object easily slides out of your hand.
Q42. medium
Grooves and rough patterns on the handles of tools and bats are made to:
Answer: A — Improve grip by increasing friction
Rough or grooved handles increase the friction between the hand and the tool, giving a firmer grip so the tool or bat does not slip out of the hand during use.
Q44. medium
A ball rolling on the ground eventually STOPS because of:
Answer: A — Friction (rolling friction and air resistance) opposing its motion
As the ball rolls, rolling friction with the ground and air resistance oppose its motion, gradually slowing it until it stops. Without friction, the ball would keep rolling forever.
Q46. medium
Which of these situations needs LESS friction?
Answer: A — A machine's rotating shaft
A rotating shaft should have low friction to turn smoothly and waste less energy - so it is lubricated. Brakes, shoe soles and tyres all NEED high friction (grip) to work safely.
Q48. medium
Ball bearings are used in the wheels of bicycles, cars and ceiling fans to:
Answer: A — Reduce friction so the parts turn smoothly
Ball bearings turn sliding friction into rolling friction, which is far smaller. This lets wheels and fan blades rotate smoothly with little friction, using less energy and wearing out slowly.
Q50. medium
Which of these best explains why sand or ash is spread on an icy road?
Answer: A — It increases friction so vehicles and people do not slip on the ice
Ice is very slippery (low friction). Spreading sand or ash roughens the surface and increases friction (grip), helping vehicles and pedestrians move safely without slipping.
Q52. medium
The soles of new shoes have deep grooves, while very old shoes become smooth. The old shoes are more slippery because:
Answer: A — The worn smooth soles provide less friction (grip)
Grooved soles grip the ground (high friction). Over time, friction wears the grooves smooth, reducing the grip. So worn, smooth-soled shoes are more slippery and less safe.
Q55. medium
Which of these reduces friction?
Answer: A — Oil, grease and ball bearings
Oil, grease (lubricants) and ball bearings reduce friction. Treads, spikes and sand on ice all INCREASE friction (grip).
Q57. medium
Rolling friction is used to advantage in:
Answer: A — Wheels and ball bearings
Wheels and ball bearings use rolling friction, which is much smaller than sliding friction, to make movement easy. Shoe soles, brakes and treads rely on high friction instead.
Q59. medium
Why is friction called a contact force?
Answer: A — It only acts when surfaces touch each other
Friction is produced by the contact between the irregularities of two touching surfaces, so it acts only when the surfaces are in contact - making it a contact force.
Q4. hard
Friction is GREATER between two ROUGH surfaces than between two SMOOTH surfaces because:
Answer: A — Rough surfaces have more/larger irregularities that interlock
Rough surfaces have more and bigger irregularities that interlock strongly, so it is harder to slide them - producing more friction. Smoother surfaces have smaller irregularities and less friction.
Q5. hard
Friction between two surfaces INCREASES when the surfaces are pressed together harder (more weight/force). This means friction depends on:
Answer: A — How hard the surfaces are pressed together
The harder two surfaces are pressed together (greater normal force/weight), the more their irregularities interlock, so friction increases. A heavier box is harder to slide than a lighter one on the same floor.
Q7. hard
The friction that acts when an object is at REST but a force tries to move it is called:
Answer: A — Static friction
Static friction acts on a stationary object when a force tries to move it. You must overcome static friction to START the object moving.
Q8. hard
The friction that acts when one surface SLIDES over another is called:
Answer: A — Sliding (kinetic) friction
Sliding friction (also called kinetic friction) acts when one object is already sliding over another. It opposes the ongoing motion.
Q9. hard
Which is generally GREATER - static friction or sliding friction?
Answer: A — Static friction (it is harder to START moving an object than to keep it moving)
Static friction is slightly greater than sliding friction. That is why it takes a bigger push to start a heavy box moving than to keep it sliding once it is already moving.
Q11. hard
For the same object and surface, which type of friction is the SMALLEST?
Answer: A — Rolling friction
Rolling friction is smaller than both static and sliding friction. This is why it is easier to roll a heavy object on wheels or rollers than to drag it - and why ball bearings are used in machines.
Q14. hard
Substances applied to reduce friction between moving parts of machines are called:
Answer: A — Lubricants (like oil, grease, graphite)
Lubricants such as oil, grease and graphite reduce friction by forming a slippery layer between surfaces. This makes machines run smoothly and reduces heat and wear on their parts.
Q15. hard
Small steel balls placed between moving parts of machines to reduce friction are called:
Answer: A — Ball bearings
Ball bearings convert sliding friction into rolling friction. The rolling balls let parts (like the axle of a wheel or a ceiling fan) turn with very little friction, so machines run smoothly.
Q17. hard
The treads (grooved patterns) on vehicle tyres are designed to:
Answer: A — Increase friction (grip) with the road, especially in wet conditions
Tyre treads increase the grip (friction) between the tyre and the road, and channel away water on wet roads. This prevents skidding. Worn, smooth (bald) tyres are dangerous because they have less grip.
Q19. hard
We can light a matchstick by striking it on a rough surface because friction:
Answer: A — Produces enough heat to ignite the match head
Rubbing (striking) the match on the rough side of the box produces heat by friction. This heat ignites the chemicals in the match head, lighting the match.
Q21. hard
Why do we need friction to WALK?
Answer: A — Friction between our feet and the ground grips them so we can push off without slipping
When we walk, our feet push backward on the ground, and friction grips them, letting us move forward. On a frictionless surface (like smooth ice), our feet would just slip and we could not walk.
Q23. hard
The brakes of a bicycle or car work by:
Answer: A — Using friction (brake pads pressing on the wheel/disc) to slow it down
Brakes work by friction: brake pads press against the moving wheel or disc, and the friction between them slows and stops the vehicle. Without friction, brakes would not work.
Q25. hard
Gymnasts apply a fine powder (chalk/magnesium carbonate) on their hands to:
Answer: A — Increase friction (grip) and absorb sweat
Gymnasts rub chalk powder on their hands to absorb sweat and increase the friction (grip) between their hands and the apparatus (like bars), so they do not slip and can hold on firmly.
Q27. hard
The frictional force exerted by liquids and gases (fluids) on objects moving through them is called:
Answer: A — Drag (fluid friction)
Fluids (liquids and gases) exert a frictional force called drag on objects moving through them. Drag opposes the motion, which is why swimming through water or cycling against the wind is harder.
Q29. hard
Why do the soles of shoes and the tyres of vehicles become SMOOTH (worn out) after long use?
Answer: A — Friction gradually wears them down
Constant rubbing against the ground causes friction, which slowly wears away the material of shoe soles and tyres. Worn (smooth) soles and tyres have less grip and must be replaced for safety.
Q31. hard
Machine parts are oiled regularly mainly to:
Answer: A — Reduce friction and wear, and prevent overheating
Oiling (lubricating) moving parts reduces friction between them. This lowers wear and tear, prevents overheating, and lets machines run smoothly and last longer.
Q33. hard
When you rub your hands together on a cold day, they get warm. This shows that friction:
Answer: A — Produces heat
Rubbing your hands overcomes friction, and the work done against friction is converted into heat, warming your hands. Friction always produces heat when surfaces rub together.
Q35. hard
It is easier to move a heavy trunk on ROLLERS (or wheels) than to drag it because rollers:
Answer: A — Change sliding friction into much smaller rolling friction
Rollers or wheels convert sliding friction into rolling friction, which is much smaller. So the trunk can be moved with far less effort on rollers than by dragging it directly on the ground.
Q37. hard
A spacecraft heats up strongly when re-entering the Earth's atmosphere because of:
Answer: A — Friction (drag) with the air producing intense heat
As a spacecraft re-enters the atmosphere at very high speed, friction (drag) with the air generates enormous heat. Spacecraft have special heat shields to protect them from burning up.
Q39. hard
Oil spilled on a road makes it dangerous for vehicles because oil:
Answer: A — Reduces friction between the tyres and the road, causing skidding
Oil acts as a lubricant, drastically reducing the friction (grip) between tyres and the road. With too little friction, vehicles skid and cannot brake or steer properly - a serious hazard.
Q41. hard
Which of these correctly ranks the three types of friction from LARGEST to SMALLEST (for the same object and surface)?
Answer: A — Static > Sliding > Rolling
For the same object and surfaces, static friction is the greatest (hardest to start moving), sliding friction is a bit less (already moving), and rolling friction is the smallest (which is why wheels help so much).
Q43. hard
Why is friction called a contact force?
Answer: A — It acts only when two surfaces are touching
Friction arises from the contact between the irregularities of two touching surfaces. It cannot act unless the surfaces are in contact, so it is a contact force (unlike gravity or magnetism).
Q45. hard
In machines, friction is reduced using lubricants and ball bearings. This is important because friction:
Answer: A — Wastes energy as heat and causes wear on the parts
Friction between moving machine parts wastes energy as heat and slowly wears the parts out. Reducing it with lubricants and ball bearings saves energy, reduces wear, and makes machines last longer and run efficiently.
Q47. hard
A key difference between fluid friction (drag) and solid friction is that fluid friction:
Answer: A — Increases as the object moves faster through the fluid
Fluid friction (drag) increases as the object's speed through the fluid increases - so a faster car or plane experiences more air resistance. This is one reason high-speed vehicles are streamlined.
Q49. hard
Which correctly matches the situation with the desired change in friction?
Answer: A — Tyre treads - increase friction; machine oil - decrease friction
Tyre treads INCREASE friction (grip) with the road for safety. Machine oil DECREASES friction between moving parts for smooth running. We change friction up or down depending on what is needed.
Q51. hard
Which statement about friction is FALSE?
Answer: A — Friction always helps and never causes any problems
Friction is a 'necessary evil' - it is essential for walking, gripping and braking, but it also wastes energy as heat and wears out surfaces. So it is FALSE that friction never causes problems. The other statements are true.
Q53. hard
Which is the correct overall summary about friction?
Answer: A — Friction opposes motion, depends on roughness and how hard surfaces press together, produces heat and wear, and can be increased or reduced as needed
Friction opposes relative motion between contacting surfaces. It depends on their roughness and how hard they press together, produces heat and wear, and is a 'necessary evil' - increased for grip (treads, spikes) or reduced for smooth motion (lubricants, ball bearings, streamlining).
Q56. hard
Which of these increases friction?
Answer: A — Making a surface rough or adding grooves
Roughening a surface or adding grooves/treads increases friction (grip). Polishing, oiling and using ball bearings all reduce friction.
Q58. hard
A person slips on a wet floor because water:
Answer: A — Reduces the friction between the feet and the floor
Water acts as a lubricant on a smooth floor, greatly reducing the friction (grip) between the feet and the floor. With too little friction, the feet slip and the person may fall.
Q60. hard
Which everyday example shows that friction produces heat?
Answer: A — Rubbing hands together to warm them, or a matchstick lighting when struck
Rubbing hands together and striking a matchstick both produce heat by friction. This heat can warm the hands or ignite the match head - clear evidence that friction generates heat.