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Practice Class 8 Science MCQs on friction, lubricants, ball bearings, matchstick ignition, fluid friction, streamlined shapes, shoe treads, and NCERT friction concepts with detailed explanations, real-life examples, and exam-oriented facts.
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Q1. Friction can be reduced by?
Correct Answer: All of the above
Detailed Explanation:
Friction is a force that opposes the motion of one surface moving over another. Although friction is useful in many situations, excessive friction can cause wear and tear, produce unwanted heat, and waste energy. Therefore, in many machines and daily-life applications, friction is reduced to improve efficiency and increase the lifespan of objects.
There are several effective methods to reduce friction. One common method is polishing surfaces. Rough surfaces contain tiny irregularities that interlock with each other, creating friction. Polishing smooths these irregularities, allowing surfaces to move more easily.
Another method is applying lubricants such as oil or grease. Lubricants form a thin layer between moving surfaces and prevent direct contact. This significantly reduces friction and minimizes wear. Machines, engines, bicycles, and industrial equipment commonly use lubricants.
Ball bearings are also widely used to reduce friction. Instead of sliding friction, ball bearings convert motion into rolling friction, which is much smaller. This is why ball bearings are found in bicycle wheels, fans, electric motors, and many machines.
Reducing friction is important because it saves energy, decreases heat generation, and improves the performance of machines. However, friction should not always be eliminated completely because certain activities, such as walking, writing, and driving, depend on friction.
Understanding methods of reducing friction helps students learn how scientific principles are applied in engineering, transportation, and everyday life.
Real-Life Examples:
- Oil is used in vehicle engines to reduce friction.
- Ball bearings are used in bicycle wheels.
- Furniture surfaces are polished to make movement easier.
Important NCERT Points:
- Friction opposes motion.
- Lubricants reduce friction.
- Polishing makes surfaces smoother.
- Ball bearings reduce sliding friction.
Exam-Oriented Fact:
Rolling friction is much less than sliding friction.
Quick Revision Line:
Friction can be reduced by polishing surfaces, using lubricants, and using ball bearings.
Q2. A matchstick catches fire because of?
Correct Answer: Friction
Detailed Explanation:
A matchstick catches fire due to friction produced when it is rubbed against the rough surface of a matchbox. Friction is a force that resists motion between two surfaces in contact. When the matchstick is struck against the matchbox, friction generates heat.
The head of a matchstick contains chemicals that are highly sensitive to heat. As the matchstick is rubbed against the striking surface, enough heat is produced to ignite these chemicals. Once ignition begins, a flame is formed and the wooden stick starts burning.
This process demonstrates the conversion of mechanical energy into heat energy. The energy supplied by rubbing the matchstick is transformed into thermal energy due to friction. When the temperature reaches the ignition point, combustion starts.
The phenomenon of frictional heating is not limited to matchsticks. Whenever two surfaces rub against each other, heat is produced. Excessive friction in machines can cause overheating, while controlled friction can be useful for generating heat.
The invention of safety matches made the process of lighting fire much safer and more convenient. Modern matchboxes are designed so that ignition occurs only when the matchstick is rubbed against a specially prepared surface.
This concept helps students understand both the advantages and practical applications of friction in daily life.
Real-Life Examples:
- Lighting a matchstick.
- Rubbing hands together to feel warmth in winter.
- Heat generation in vehicle brakes.
Important NCERT Points:
- Friction produces heat.
- Matchstick chemicals ignite due to heat.
- Mechanical energy converts into heat energy.
- Controlled friction can be useful.
Exam-Oriented Fact:
The heat generated by friction is sufficient to ignite the chemicals on a matchstick head.
Quick Revision Line:
A matchstick catches fire because friction generates heat during rubbing.
Q3. The shape of aeroplanes is streamlined to?
Correct Answer: Reduce fluid friction
Detailed Explanation:
Aeroplanes are designed with streamlined shapes to reduce fluid friction, also known as air resistance or drag. Fluid friction occurs when an object moves through a fluid such as air or water. The faster the object moves, the greater the fluid friction acting against it.
A streamlined shape allows air to flow smoothly around the body of the aeroplane. This reduces turbulence and minimizes resistance. As a result, the aircraft can move faster, consume less fuel, and operate more efficiently.
If aeroplanes had flat or irregular shapes, they would experience much greater air resistance. The engines would need to produce more power to overcome this resistance, increasing fuel consumption and reducing efficiency.
The concept of streamlined design is widely used in transportation and engineering. Birds, fish, submarines, racing cars, and high-speed trains often have streamlined bodies because nature and science both show that such shapes reduce resistance.
Fluid friction is an important topic because it affects motion through air and water. Engineers carefully study aerodynamics when designing aircraft to ensure safety, speed, and fuel efficiency.
Understanding streamlined shapes helps students connect scientific principles with real-world technology and transportation systems.
Real-Life Examples:
- Aeroplanes have pointed noses and smooth bodies.
- High-speed trains use streamlined designs.
- Fish naturally possess streamlined bodies.
Important NCERT Points:
- Fluid friction is also called drag.
- Streamlined shapes reduce air resistance.
- Less drag improves speed and efficiency.
- Aeroplanes are designed using aerodynamic principles.
Exam-Oriented Fact:
A streamlined body minimizes turbulence and reduces fluid friction.
Quick Revision Line:
Aeroplanes are streamlined to reduce fluid friction and improve efficiency.
Q4. Soles of shoes are treaded to?
Correct Answer: Increase friction for better grip
Detailed Explanation:
The soles of shoes are provided with grooves, patterns, or treads to increase friction between the shoe and the ground. Friction is essential for walking, running, and maintaining balance because it prevents slipping.
When a person walks, the foot pushes backward against the ground, and friction provides the forward force needed for movement. If friction is too low, the foot may slip and cause accidents. Treaded soles improve grip by increasing contact with uneven surfaces and preventing slipping.
The grooves in shoe soles also help channel away water on wet surfaces. Without these grooves, a thin layer of water could reduce friction significantly, making the surface slippery.
Different types of footwear are designed with specific tread patterns depending on their use. Sports shoes, hiking boots, and safety footwear often have deeper treads to provide better traction on rough or slippery surfaces.
The importance of friction becomes evident when walking on ice or wet marble floors, where the friction is very low. Treaded soles help increase safety and stability under such conditions.
This concept demonstrates how friction, although often considered a resisting force, is actually necessary for many everyday activities.
Real-Life Examples:
- Sports shoes have deep grooves for better grip.
- Hiking boots provide traction on rocky surfaces.
- Vehicle tyres also use treads to increase friction.
Important NCERT Points:
- Friction helps us walk safely.
- Treads increase grip.
- Grooves reduce slipping on wet surfaces.
- Low friction can cause accidents.
Exam-Oriented Fact:
Tyres and shoe soles are treaded for the same reason—to increase friction.
Quick Revision Line:
Shoe soles are treaded to increase friction and provide better grip.
Q5. Which of the following surfaces has the least friction?
Correct Answer: Wet marble floor
Detailed Explanation:
Among the given options, a wet marble floor has the least friction. Friction depends on the nature of the surfaces in contact. Smooth surfaces generally produce less friction than rough surfaces, and the presence of water can reduce friction even further.
Marble is naturally smooth. When water is present on the marble surface, it forms a thin layer between the floor and the feet or shoes. This layer acts somewhat like a lubricant, reducing direct contact between the two surfaces and lowering friction.
Because of this reduced friction, wet marble floors become extremely slippery and can cause people to lose balance and fall. This is why warning signs are often placed near freshly cleaned floors in public places.
In contrast, sandpaper, carpets, and concrete roads have rough surfaces that create much higher friction. Their rough texture increases resistance to motion and provides better grip.
Understanding surfaces with low and high friction is important for safety, engineering, and transportation. Engineers design roads, footwear, and tyres while considering frictional properties to ensure safe movement.
This topic helps students understand how friction varies depending on surface texture and environmental conditions.
Real-Life Examples:
- Wet marble floors are slippery after cleaning.
- Ice-covered roads become dangerous due to low friction.
- Oil spills reduce friction on roads and factory floors.
Important NCERT Points:
- Smooth surfaces have less friction.
- Water can reduce friction.
- Rough surfaces provide better grip.
- Low friction increases the risk of slipping.
Exam-Oriented Fact:
A wet marble floor is more slippery than a dry marble floor because water reduces friction.
Quick Revision Line:
Wet marble floors have the least friction because water reduces contact between surfaces.
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