Science Notes for Chapter 9 Friction Class 8 - FREE PDF Download
FAQs on Friction Class 8 Science Chapter 9 CBSE Notes - 2025-26
1. What is the basic concept of friction as explained in Class 8 Science, Chapter 9?
Friction is a contact force that opposes the motion or attempted motion of one object over another. For revision, remember that it always acts in the direction opposite to the movement. For example, when you push a book across a table, friction is the force exerted by the table on the book, slowing it down.
2. What are the key factors affecting friction that I should recap from these notes?
For a quick revision of Chapter 9, focus on these two main factors:
- The nature of the surfaces: Rougher surfaces have more irregularities, which interlock and create greater friction. Smoother surfaces produce less friction.
- The force pressing the surfaces together: The harder two surfaces are pressed against each other, the greater the force of friction. This is why it's harder to push a heavy box than a light one.
3. What are the main types of friction to summarise for revision?
The four key types of friction to remember for your summary are:
- Static Friction: The force that must be overcome to start an object moving from rest.
- Sliding Friction: The force that opposes motion when one object is sliding over another.
- Rolling Friction: The force that opposes motion when an object rolls over a surface (e.g., a wheel). It is significantly smaller than sliding friction.
- Fluid Friction (Drag): The resistive force exerted by fluids (liquids or gases) on an object moving through them.
4. Why is friction often called a 'necessary evil'?
Friction is called a 'necessary evil' because it has both crucial advantages and significant disadvantages. It is necessary because without it, we couldn't walk, write, or stop a moving car. It is an evil because it causes wear and tear on machine parts, wastes energy by producing unwanted heat, and slows down motion when we don't want it to.
5. How does a streamlined shape help reduce friction?
A streamlined shape, like that of a fish, bird, or aeroplane, is designed to reduce fluid friction or drag. It allows the fluid (air or water) to flow smoothly around the object's body with minimal disturbance. This minimises the resistance, allowing the object to move through the fluid more efficiently and with less energy expenditure.
6. For a quick revision, what is the order of friction types from highest to lowest?
A key concept to remember for exams is the relative magnitude of friction types. For the same pair of surfaces, the order from highest to lowest is: Static Friction > Sliding Friction > Rolling Friction. This is because it takes more force to get an object to start moving than to keep it sliding, and rolling requires the least force of all.
7. How do lubricants work to reduce friction?
Lubricants are substances like oil or grease that reduce friction between moving parts. When a lubricant is applied between two surfaces, it forms a thin layer that separates them. This prevents the microscopic irregularities on the surfaces from interlocking directly. As a result, the surfaces can slide over the lubricant layer smoothly, significantly reducing sliding friction.
8. In what ways is friction intentionally increased in everyday life?
While we often try to reduce friction, it is sometimes increased on purpose for safety and functionality. Key examples to recap are:
- Tyre Treads: The grooved patterns on tyres increase friction with the road, providing a better grip and preventing skidding, especially on wet surfaces.
- Brake Pads: In vehicles, brake pads are designed with high-friction materials to press against the wheels, converting motion into heat and safely slowing the vehicle down.
- Shoe Soles: The soles of shoes are designed to be rough to increase friction with the ground, preventing us from slipping.
9. Why is it much easier to roll a suitcase than to drag it?
It is easier to roll a suitcase because rolling friction is much less than sliding friction. When you drag the suitcase, its entire bottom surface slides against the ground, causing significant resistance. However, when you use its wheels, the contact points are very small, and the mechanism of rolling generates far less opposition to motion. This concept demonstrates the practical benefit of converting sliding motion to rolling motion to reduce friction.
