CBSE Class 8 Science Chapter-9 Important Questions - Free PDF Download
FAQs on Important Questions for CBSE Class 8 Science Chapter 9 - Friction
1. What are the most important types of questions to prepare from CBSE Class 8 Science Chapter 9, Friction, for the 2025-26 exams?
For Chapter 9, Friction, students should focus on a mix of question types to score well. Based on previous exam patterns, important questions include:
- Conceptual Questions: Such as explaining why friction is called a 'necessary evil'.
- Reasoning-Based Questions: For instance, 'Why is it difficult to walk on a wet marble floor?'.
- Differentiation Questions: A very common question is to differentiate between static, sliding, and rolling friction.
- Application-Based Questions: Explaining how lubricants work or why vehicle tyres are treaded.
- Definitions: Key terms like drag, fluid friction, and lubricants are often asked as 1-mark questions.
2. What is the fundamental cause of friction between two surfaces?
The fundamental cause of friction is the presence of microscopic irregularities (roughness) on the surfaces that are in contact. Even surfaces that appear very smooth to the naked eye have tiny hills and valleys. When one surface moves over another, these irregularities interlock with each other. Friction is the force that must be overcome to break this interlocking and allow movement.
3. Explain with examples how friction can be increased and decreased.
Friction can be modified depending on the requirement. Here are examples for both:
- Increasing Friction: This is done to improve grip and prevent slipping. For example, the soles of shoes are grooved and the tyres of vehicles are treaded. Gymnasts also apply a coarse substance on their hands for a better grip on the apparatus.
- Decreasing Friction: This is done to make movement smoother and more efficient. For example, we sprinkle fine powder on a carrom board to allow the striker and coins to move easily. In machines, lubricants like oil or grease are applied between moving parts to reduce wear and tear.
4. Why is friction often described as a 'necessary evil'? Explain with examples.
Friction is called a 'necessary evil' because while it is essential for many of our daily activities, it also has undesirable effects.
- Why it is Necessary: We cannot walk, write, or drive without friction. It is the friction between our feet and the ground that allows us to push off and move forward. Friction helps a car's tyres grip the road and allows us to hold objects.
- Why it is an Evil: Friction opposes motion, which means extra energy is wasted in overcoming it. It also causes wear and tear on moving parts, like the soles of our shoes or the components of an engine. Furthermore, friction produces heat, which can damage machinery if not managed.
5. What is the difference between static, sliding, and rolling friction?
Static, sliding, and rolling friction are three types of contact friction:
- Static Friction: This is the friction that acts on an object when it is at rest. It is the force you must overcome to start moving an object. It is the strongest of the three.
- Sliding Friction: This is the friction that acts when one surface slides over another. It is slightly less than static friction, which is why it's often easier to keep an object moving than to start it moving.
- Rolling Friction: This is the friction that acts when an object, like a wheel or a ball, rolls over a surface. It is the weakest of the three, which is why it is much easier to move heavy objects using wheels.
In general, Static Friction > Sliding Friction > Rolling Friction.
6. If rolling friction is the weakest, why do bicycle brakes use sliding friction to stop the wheel?
This is an excellent question that highlights the different purposes of friction. While rolling friction is minimized to allow the bicycle to move efficiently with less effort, a strong opposing force is needed to stop it. Brake pads are designed to press against the wheel rim, creating a large amount of sliding friction. This strong frictional force effectively opposes the wheel's motion, converting kinetic energy into heat and bringing the bicycle to a quick and safe stop.
7. What is fluid friction and how is its effect minimised in objects like aeroplanes and ships?
Fluid friction, also known as drag, is the frictional force exerted by fluids (like air and water) on objects moving through them. To minimise this resistance and save energy, objects are given a special shape called a streamlined shape. This shape, often pointed at the front and tapered at the back like a fish or a bird, allows the fluid to flow smoothly around the object, significantly reducing the effect of drag. This is why aeroplanes, rockets, ships, and even racing cars have streamlined bodies.
8. Can friction ever be completely eliminated between two surfaces? Explain why or why not.
No, friction can never be completely eliminated. Even the most polished and smoothest surfaces have microscopic irregularities. While we can use lubricants or polishing to reduce the effect of these irregularities, we cannot remove them entirely. Therefore, some amount of frictional force will always exist when two surfaces are in contact. We can only reduce friction to a very low value, but we can never make it zero.
9. How do lubricants like oil and grease work to reduce friction in machine parts?
Lubricants are substances that reduce friction between moving surfaces. When a lubricant like oil or grease is applied between two machine parts, it forms a thin, slippery layer. This layer fills the microscopic irregularities on the surfaces and keeps them from directly interlocking with each other. As a result, the parts slide over the smooth lubricant layer instead of rubbing against each other, which drastically reduces friction, heat generation, and wear and tear.
10. A book gently pushed on a table moves a short distance and then stops. Which force is responsible for this, and what does it depend on?
The force responsible for stopping the book is sliding friction. This force acts in the direction opposite to the book's motion. The magnitude of this frictional force depends on two main factors:
- The nature of the surfaces in contact: A rougher table surface would create more friction and stop the book sooner.
- The mass of the book: A heavier book would press down on the table with more force, increasing the interlocking of irregularities and thus increasing the frictional force.
