Answer: Friction is a force that opposes the relative motion or the tendency of motion between two surfaces in contact. For example, friction between our shoes and the ground helps us to walk without slipping.

Answer: Friction arises only when two surfaces actually touch each other. Since it acts due to physical contact between surfaces, it is called a contact force.

Answer: Friction always acts in a direction opposite to the direction of motion or attempted motion of an object. It tries to slow down or stop the motion.

Answer: Without friction, our feet would slip on the ground and we would not be able to walk properly. Every step would make us slide instead of moving forward smoothly.

Answer: The floor and air offer frictional forces against the moving ball. These forces oppose its motion, continuously reduce its speed, and finally bring it to rest.

Answer: The main cause of friction is the interlocking of microscopic irregularities present on the surfaces in contact. Even smooth-looking surfaces have tiny bumps and hollows that lock together and resist motion.

Answer: Rough surfaces have more and larger irregularities. Due to greater interlocking between surfaces, friction is greater on rough surfaces than on smooth surfaces.

Answer: Friction mainly depends on: (i) The nature of surfaces in contact (whether they are rough or smooth). (ii) The normal force, i.e., how hard the surfaces are pressed against each other.

Answer: When the normal force increases, surfaces press more firmly against each other, so their irregularities interlock more strongly. As a result, the frictional force between them increases.

Answer: Even surfaces that appear very smooth to our eyes still have tiny bumps and hollows when seen under a microscope. These microscopic irregularities cause friction, proving that perfectly smooth surfaces do not exist in real life.

Answer: Static friction is the frictional force that acts on a body at rest when a force is applied to move it, but it does not move. For example, when we push a heavy box and it does not move initially, static friction balances our push.

Answer: Limiting friction is the maximum value of static friction just before the object starts to move. Beyond this limit, the object begins to slide and static friction changes into sliding friction.

Answer: Sliding friction is the frictional force that acts when one surface actually slides over another surface. For example, when a book is pushed across a table and it moves, it experiences sliding friction with the table surface.

Answer: During sliding, surfaces do not get time to interlock fully because of continuous motion. This partial interlocking reduces the frictional force, so sliding friction is usually smaller than static friction for the same surfaces.

Answer: Rolling friction is the frictional force that acts when a body rolls over a surface, such as a wheel rolling on the road. It is much smaller than sliding friction because only a small part of the wheel touches the surface at a time and interlocking is less.

Answer: For the same pair of surfaces, the order is: Static friction > Sliding friction > Rolling friction.

Answer: Wheels change sliding friction into rolling friction. Since rolling friction is much smaller than sliding friction, it becomes easier to move a suitcase with wheels than dragging it without wheels.

Answer: When we walk, our feet push the ground backward. Due to friction between the ground and our shoes, the ground pushes our feet forward, allowing us to move. Without friction, our feet would simply slip backward instead of moving us ahead.

Answer: When brakes are applied, brake pads press against the rotating wheels. Friction between the brake pads and wheels opposes their motion, reduces their speed and finally brings the vehicle to a stop safely.

Answer: While writing, friction between the pencil lead and the paper causes small particles of graphite to stick to the paper, forming letters. Thus, friction between the pencil and paper helps us to write clearly.

Answer: When we tie a knot, the surfaces of the rope or lace rub against each other. Friction between these surfaces prevents them from sliding apart easily, so the knot remains tight and secure.

Answer: (i) Friction between tyres and road helps vehicles to move and turn without skidding. (ii) Friction between our hands and objects allows us to hold and grip them firmly without slipping.

Answer: Friction is undesirable because it causes wear and tear of machine parts, wastes energy as heat, and reduces the efficiency of machines and vehicles. Extra fuel is required to overcome friction in moving parts.

Answer: In a machine, metal parts rub against each other while moving. Friction between these surfaces scrapes off tiny particles over time, gradually wearing away the surfaces and damaging the parts if not maintained properly.

Answer: When two surfaces rub, part of the mechanical energy of motion is converted into heat energy due to friction. This heat often does not perform any useful work and is lost to the surroundings, leading to wastage of energy.

Answer: Lighting a matchstick is an example. When the match head is rubbed against the rough side of the matchbox, friction produces heat which ignites the chemicals on the match head, causing it to burn.

Answer: In engines, many parts like pistons and shafts move rapidly and rub against each other. The friction between these moving parts produces a lot of heat, so the engine becomes hot during long drives.

Answer: Lubricants are substances used to reduce friction between moving surfaces by making them smoother. Examples include oil and grease, which are commonly used in machines and vehicles.

Answer: Lubricants fill the gaps between irregularities of surfaces, forming a thin, smooth layer in between. This reduces the direct contact and interlocking between surfaces, thereby decreasing friction and allowing smoother movement.

Answer: Ball bearings contain small metal balls that roll between moving parts. They change sliding friction into rolling friction, which is much less. This reduces friction and allows shafts and wheels to rotate smoothly with less effort.

Answer: Polishing makes a surface smoother by removing small irregularities. As a result, interlocking of surfaces is reduced and the frictional force between them decreases, so objects can move more easily over the polished surface.

Answer: (i) Roughening surfaces, for example by making grooves on tyres and the soles of shoes. (ii) Using materials with higher friction, such as rubber on brakes and tyres to provide better grip and control.

Answer: Grooves on tyres increase friction between the tyre and road surface, especially on wet or slippery roads. This helps in better grip, preventing skidding and allowing safe acceleration and braking.

Answer: Fluid friction is the frictional force exerted by fluids (liquids and gases) on objects moving through them. It is also called drag. Both air and water exert fluid friction on moving objects.

Answer: Fluid friction depends on: (i) The speed of the object moving through the fluid. (ii) The shape and size of the object. (iii) The nature (thickness or viscosity) of the fluid itself.

Answer: Drag is the resistive force exerted by a fluid (like air or water) on a body moving through it. It always acts in a direction opposite to the motion and tries to slow down the moving body.

Answer: Cars and aeroplanes are given streamlined shapes to reduce air resistance (drag) while moving at high speed. Less drag means they can move faster with less fuel, improving efficiency and stability.

Answer: A streamlined body is narrow at the front, broader in the middle and tapered at the back. This shape allows fluids to flow smoothly around it, reducing drag. A fish is a natural example of a streamlined body in water.

Answer: Water exerts fluid friction on the moving boat, opposing its motion. Greater fluid friction slows the boat down, and more force (or engine power) is required to maintain high speed against this resistance.

Answer: Friction is necessary because it helps in walking, writing, gripping, and many useful activities. At the same time, it causes wear and tear, heating and energy loss. Since we cannot do without it, but also try to reduce it, it is called a “necessary evil”.

Answer: On a wet floor, a thin layer of water reduces friction between our feet and the floor surface. With less frictional grip, our feet cannot hold the ground firmly, so we tend to slip more easily than on a dry floor where friction is higher.

Answer: Oily or highly polished surfaces are very smooth, so friction between the surface and our footwear is greatly reduced. With little friction, our feet cannot get enough grip, making walking difficult and unsafe.

Answer: New shoes may have very smooth soles that provide less friction with the ground. Roughening them increases friction and improves grip, reducing the chance of slipping while walking or running.

Answer: Chalk powder absorbs sweat and increases friction between their hands and the apparatus (bar, rod, weights). This improved grip helps prevent slipping and allows them to perform safely and correctly.

Answer: A rough grip on a pen or pencil increases friction between our fingers and the writing instrument. This prevents slipping and makes it easier and more comfortable to hold and control while writing for a long time.

Answer: Brakes work by creating friction. With use, brake pads wear out and may not produce enough friction to stop the vehicle effectively. Regular checking and maintenance ensure proper friction for safe braking and prevent accidents.

Answer: A bowler uses friction between the ball and fingers to grip and spin the ball. A batsman uses friction between the bat handle and hands to hold the bat firmly. In both cases, friction provides control and accuracy during play.

Answer: The side of the matchbox has a rough surface coated with chemicals. When the match head is rubbed against this surface, friction produces heat, which ignites the chemicals on the match head, causing the stick to catch fire.

Answer: Spikes dig into the track surface and increase friction between the shoes and the ground. This provides a strong grip, prevents slipping, and allows athletes to run faster and with more stability during races.

Answer: Rubber tyres provide better friction with the road surface than metal tyres. This improved friction gives good grip, allows smooth riding, better braking, and reduces slipping, making cycling safer and more comfortable.