Extra 20 important long-answer questions with detailed answers from Chapter:9 Friction (Class 8 Science - CBSE):-
1. What is friction? Explain its causes.
Answer:
Friction is the force that opposes the relative motion of two surfaces in contact.
Causes of Friction:
Surface Roughness
– The irregularities on surfaces interlock, causing resistance.
Intermolecular Forces
– Attraction between molecules of two surfaces increases friction.
Nature of the Surface
– Rough surfaces create more friction than smooth surfaces.
Weight of the Object
– Heavier objects press surfaces together, increasing friction.
Contact Area
– A larger contact area increases friction.
2. Explain the different types of friction.
Answer:
Types of Friction:
Static Friction
– The force that prevents an object from moving when force is applied.
Sliding Friction
– The force that resists motion when an object
slides
over another.
Rolling Friction
– The force that opposes motion when an object rolls over a surface.
Fluid Friction (Drag)
– The resistance experienced by objects moving in liquids or gases.
Order of Friction: Static > Sliding > Rolling > Fluid
3. Why is friction considered both a friend and a foe?
Answer:
Friction as a Friend:
Helps in walking by preventing slipping.
Enables us to hold objects.
Helps in writing with a pen or pencil.
Allows vehicles to stop when brakes are applied.
Produces heat that helps light a matchstick.
Friction as a Foe:
Causes wear and tear in machines.
Produces heat in machines, reducing efficiency.
Requires more energy to move objects.
Reduces the lifespan of vehicle tyres.
Causes blisters when rubbing against the skin.
4. How can friction be increased? Give examples.
Answer:
Ways to Increase Friction:
By Making Surfaces Rough
– Example: Sand is spread on icy roads to improve grip.
By Increasing the Contact Area
– Example: Wider soles in sports shoes.
By Using Treads and Grooves
– Example: Tyres have grooves to increase friction on roads.
By Applying More Force
– Example: Pressing harder while writing increases grip.
By Using Dry Surfaces
– Example: Gymnasts use powder to increase grip.
5. How can friction be reduced? Give examples.
Answer:
Ways to Reduce Friction:
By Using Lubricants
– Example: Oil and grease are used in machines.
By Using Ball Bearings
– Example: Ball bearings in fans and bicycles reduce rolling friction.
By Making Surfaces Smooth
– Example: Polishing surfaces reduces friction.
By Using Air Cushions
– Example: Hovercrafts reduce friction using air layers.
By Using Streamlined Shapes
– Example: Aeroplanes and cars have streamlined bodies to reduce air resistance.
6. Why is rolling friction less than sliding friction?
Answer:
Reasons Why Rolling Friction is Less:
Less Surface Contact
– Rolling reduces the area in contact with the surface.
Less Interlocking
– Rough surfaces do not interlock as much in rolling motion.
Lesser Resistance
– Rolling allows smoother motion compared to sliding.
Use of Ball Bearings
– Helps in reducing friction in wheels and machines.
Practical Uses
– Wheels, roller skates, and conveyor belts use rolling friction for efficiency.
7. What is fluid friction? How can it be reduced?
Answer:
Fluid friction (drag) is the resistance experienced by objects moving through fluids (liquids or gases).
Ways to Reduce Fluid Friction:
Streamlining
– Example: Birds and fish have streamlined bodies.
Using Special Coatings
– Example: Ships have smooth coatings to reduce drag.
Reducing Surface Area Facing the Fluid
– Example: Cyclists bend forward to reduce air resistance.
Using Proper Materials
– Example: Swimmers use sleek, tight suits.
Optimizing Speed
– Reducing excessive force to avoid turbulence.
8. How does friction help in walking?
Answer:
Static friction between the foot and the ground prevents slipping.
As we push backward, friction exerts an opposite force, helping us move forward.
If friction were absent, our feet would slip, making walking impossible.
Rubber soles on shoes increase friction for a better grip.
On smooth or wet surfaces, friction is reduced, making walking difficult.
9. How do lubricants help in reducing friction?
Answer:
Lubricants form a thin layer between surfaces, preventing direct contact.
They reduce the interlocking of rough surfaces.
Examples: Oil, grease, and powders in machines and car engines.
Reduces wear and tear, increasing machine efficiency.
Helps in smooth functioning of moving parts like gears and chains.
10. Why do gymnasts and weightlifters use powder on their hands?
Answer:
Powder absorbs moisture and increases grip.
It increases friction between hands and the equipment.
Prevents slipping while performing exercises.
Helps in better control and stability.
Used in activities like gymnastics, rock climbing, and weightlifting.
11. Why do car tyres have grooves?
Answer:
Grooves increase friction between the tyre and the road.
Prevents skidding, especially on wet surfaces.
Improves vehicle control and safety.
Allows water to escape from under the tyres.
Worn-out tyres reduce friction, making driving dangerous.
12. Why do space shuttles experience high friction when entering Earth’s atmosphere?
Answer:
High-speed motion creates intense air resistance.
Friction generates extreme heat due to air compression.
The outer surface of the shuttle experiences high temperatures.
Special heat-resistant materials are used to protect the shuttle.
Without protection, the spacecraft could burn up due to friction.
13. Why do vehicles use brakes to stop?
Answer:
Brakes create friction between the brake pads and wheels.
This friction opposes motion and slows the vehicle.
The greater the friction, the quicker the vehicle stops.
Worn-out brakes reduce friction, making stopping difficult.
Brakes can overheat due to excessive friction.
14. Why do matchsticks catch fire when rubbed on a rough surface?
Answer:
Friction between the matchstick head and the surface generates heat.
The heat ignites chemicals on the match head.
The flame starts burning due to this ignition.
A smooth surface won’t generate enough heat to light the match.
This principle is used in fire-starting techniques.
15. How does friction affect the efficiency of machines?
Answer:
Friction causes
wear and tear
of machine parts, reducing their lifespan.
It
produces heat
, which can damage machine components.
Increased friction
reduces efficiency
as more energy is lost in overcoming resistance.
It requires
extra force or energy
to keep machines running.
Lubricants are used to
reduce friction
and improve efficiency.
Excess friction may lead to
breakdown of machines
if not controlled.
16. What is static friction? How is it different from sliding friction?
Answer:
Static Friction:
It is the force that prevents an object from starting to move.
It is the
strongest
type of friction.
It acts when a force is applied but the object does not move.
Example: A heavy box remains stationary even if we push it lightly.
Sliding Friction:
It acts when an object
slides
over another surface.
It is
less than static friction
but more than rolling friction.
It slows down the object while in motion.
Example: A book sliding on a table stops due to sliding friction.
17. What are some examples where friction is useful in daily life?
Answer:
Walking
– Friction between the ground and our feet prevents slipping.
Writing
– Friction between paper and pen helps in writing.
Braking in Vehicles
– Friction between the tyres and road helps in stopping cars.
Holding Objects
– Friction helps us grip objects firmly.
Lighting a Matchstick
– Rubbing the match against the rough surface produces heat and ignites it.
Sports Activities
– Friction provides grip to players in games like football and cricket.
18. Why do aeroplanes and ships have streamlined shapes?
Answer:
To Reduce Fluid Friction
– A streamlined body allows smooth movement through air or water.
To Improve Speed
– Less resistance helps in moving faster.
To Save Fuel
– Reduced friction decreases fuel consumption.
To Avoid Drag
– Sharp and curved edges minimize air and water resistance.
To Ensure Stability
– The shape helps in maintaining balance while moving.
Example:
Birds and fish also have streamlined bodies to move efficiently.
19. How does friction help in sharpening a knife?
Answer:
When a knife is rubbed against a rough stone,
friction removes the dull edges
.
The rough surface
grinds away small metal particles
, making the blade sharper.
Heat is produced
, which smoothens the new sharp edge.
This process is called
honing or sharpening
.
Regular sharpening maintains
cutting efficiency
.
Too much sharpening can
wear out the blade
over time.
20. Why do tyres of racing cars have fewer grooves than normal car tyres?
Answer:
Racing cars move at
high speeds
, so excessive friction slows them down.
Fewer grooves reduce rolling friction
, allowing them to move faster.
They provide a
better grip on dry tracks
, preventing skidding.
On wet roads, deep grooves are needed to
drain water
and prevent slipping.
Racing cars use
special tyres for wet and dry conditions
.
In normal cars,
more grooves increase safety
on different road surfaces.
