Answer:

Definition: Friction is a force that opposes the relative motion or the tendency of motion between two surfaces in contact.

Friction as a contact force:

• Friction arises only when two surfaces are in physical contact.
• If a body is not touching another surface, friction between them is zero.
• Therefore, friction is classified as a contact force, unlike gravitational or magnetic forces which act from a distance.

Main cause of friction:

• Even surfaces that look smooth to our eyes have tiny irregularities (bumps and hollows) when seen under a microscope.
• When two surfaces are in contact, these irregularities of one surface interlock with the irregularities of the other surface.
• When one surface tries to move over the other, these interlocking irregularities oppose the motion, giving rise to friction.

Thus, friction is caused mainly due to interlocking of microscopic irregularities present on the surfaces in contact.

Answer:

Activity to show friction opposes motion:

1. Take a book and slide it gently on a table.
2. Observe that the book gradually slows down and finally comes to rest, even though no one is touching it.
3. This shows that some force opposite to the direction of motion is acting on the book.
4. That opposing force is the friction between the book and the table surface.

Effect on speed of a moving object:

• Friction always acts in a direction opposite to motion.
• It reduces the speed of moving objects by converting part of their kinetic energy into heat.
• If no external force is applied to keep the object moving, friction will finally bring the object to rest.

Hence, friction opposes motion, slows down moving bodies and, if unbalanced, eventually stops them.

Answer:

(i) Effect of nature of surfaces:

• Rough surfaces have more and larger microscopic irregularities.
• When two rough surfaces come in contact, their irregularities interlock strongly.
• This strong interlocking produces greater friction.
• Smooth surfaces have smaller irregularities, so interlocking is less, and therefore friction is smaller.

(ii) Effect of force with which surfaces are pressed:

• The force with which two surfaces are pressed together is called the normal force.
• If normal force increases, surfaces are pressed more tightly, so the irregularities interlock more strongly.
• This increases the frictional force between them.

Thus, friction depends on both the nature of surfaces in contact and the normal force pressing them together.

Answer:

Definition of static friction:

Static friction is the frictional force that acts on a body at rest when an external force is applied to move it but it has not yet started moving.

Activity:

1. Place a block or a heavy book on a table.
2. Gently push it with a small force. The block does not move.
3. Increase the force little by little. The block still does not move.
4. This means a frictional force equal and opposite to your push is acting on the block.
5. Finally, when you apply a sufficiently large force, the block just starts to move.

Limiting friction:

• The maximum value of static friction just before the body starts to move is called limiting friction.
• Up to this limit, static friction adjusts itself to exactly balance the applied force.

Hence, static friction is a self-adjusting force that increases with applied force until it reaches a maximum value called limiting friction, after which motion begins.

Answer:

Sliding friction: Sliding friction is the frictional force that acts when one surface slides over another surface. Example: a book sliding on a table.

Rolling friction: Rolling friction is the frictional force that acts when an object rolls over a surface. Example: a wheel or a ball rolling on the ground.

Why rolling friction is much smaller:

• In sliding friction, a large area of one surface is in direct contact with the other at each moment, leading to more interlocking of irregularities.
• In rolling friction, only a small part of the wheel or ball touches the surface at any instant.
• The point of contact keeps changing, so there is less interlocking between irregularities.
• Deformation (bending) of surfaces near the contact region is also less in rolling motion.

Due to reduced contact and interlocking, rolling friction is much smaller than sliding friction, which is why wheels and rollers are widely used in transport and machines.

Answer:

Order of magnitude:

For the same pair of surfaces, the frictional forces follow the order: Static friction > Sliding friction > Rolling friction.

Explanation with examples:

• Static friction: When we try to push a heavy table, it does not move at first. The frictional force that opposes our push is static friction. It is usually the highest because surfaces are fully interlocked.
• Sliding friction: Once the table starts moving, it is a little easier to keep it sliding than to start it. This is because sliding friction is less than static friction; the surfaces do not interlock as strongly while sliding.
• Rolling friction: If we place the table on rollers or wheels, pushing it becomes much easier. Now the friction is rolling friction, which is the least among the three.

Thus, for the same surfaces, starting motion requires maximum effort (static friction), maintaining sliding motion requires less effort (sliding friction), and rolling motion needs the least effort (rolling friction).

Answer:

Useful effects of friction:

1. Walking: Friction between our feet (or shoes) and the ground provides the grip needed for walking. Without it, our feet would slip and we would not be able to move forward properly.
2. Writing: Friction between a pen or pencil and paper allows ink or graphite to stick to the paper, enabling us to write and draw.
3. Holding objects: Friction between our fingers and objects helps us to hold and grip them firmly. Smooth or oily objects are difficult to hold because friction is low.
4. Running and sports: Athletes depend on friction between their shoes and the ground to get a good grip. Spikes and studs increase friction, allowing quick starts, stops and turns without slipping.
5. Movement and stopping of vehicles: Friction between tyres and road allows vehicles to move, turn and stop safely. Brakes use friction to slow down or stop moving vehicles.
6. Tying knots and fastening screws: Friction between threads of a screw or surfaces of a rope keeps knots and screws tight.

Why we cannot live comfortably without friction:

• Without friction, walking, writing, holding objects, driving vehicles, tying laces or even sitting on a chair would become extremely difficult.
• Every step would cause slipping, vehicles would not move properly or stop, and most machines would not work as intended.

Therefore, friction plays a very important and useful role in our daily life, making it impossible for us to live comfortably without it.

Answer:

(a) Walking:

• When we walk, we push the ground backward with our feet.
• Due to friction between our shoes and the ground, the ground exerts an equal and opposite force on our feet.
• This frictional force helps us to move forward without slipping.

(b) Writing:

• When a pencil or pen is moved on paper, friction between the tip and paper resists the motion.
• This resistance helps graphite (from pencil) or ink (from pen) to get deposited on the paper.
• Thus, friction allows letters and figures to be formed while writing.

(c) Driving a bicycle or car:

• Friction between the tyres and the road provides the necessary grip for the vehicle to move forward when the wheels rotate.
• Friction in brakes (between brake shoes and wheel) helps to slow down or stop the vehicle.
• Without sufficient friction, tyres would skid, making driving unsafe.

In all these cases, friction is essential for getting grip, control and safety in our movements and activities.

Answer:

Why it is necessary:

• Friction enables us to walk, run and stand without slipping.
• It allows vehicles to move safely and brakes to function effectively.
• We can write, hold things, tie knots and use tools due to friction.
• Many machines like belts and pulleys work only because of friction between their parts.

Why it is an evil:

• Friction causes wear and tear of moving parts like tyres, shoes, machine components etc.
• It produces heat in machines, which can damage them and waste energy.
• Extra energy or fuel is required to overcome friction, reducing efficiency and increasing cost.
• Friction sometimes causes unwanted noise and vibration.

Conclusion:

We cannot completely remove friction because it is very useful and essential in many activities, but we also try to reduce it where it causes harm. Therefore, friction is called a necessary evil.

Answer:

Wear and tear in machines:

• In machines, metal parts like gears, shafts and bearings rub against each other during motion.
• Due to friction, tiny particles are scraped off from these surfaces each time they move.
• Over a long period, this continuous scraping leads to wear and tear of parts, causing loosening, damage or breakdown of machines.

Energy loss due to heat:

• When surfaces rub, friction converts part of the mechanical energy into heat.
• This heat often does not do any useful work and is lost to the surroundings.
• Machines then require more energy or fuel to perform the same amount of work.

Methods to reduce harmful effects:

• Lubrication: Applying oil, grease or other lubricants between moving parts reduces direct contact and friction.
• Polishing: Making surfaces smoother by polishing decreases irregularities and friction.
• Use of ball bearings: Ball bearings convert sliding friction into rolling friction, significantly reducing wear and tear.
• Proper design and maintenance: Using suitable materials and regular servicing helps keep friction under control.

Thus, while friction causes wear, tear and energy loss in machines, these effects can be reduced by suitable engineering methods.

Answer:

Lubricants:

Substances used to reduce friction between surfaces in contact are called lubricants. Common lubricants are oil, grease and graphite.

How lubricants reduce friction:

• Machine parts like gears and axles have tiny irregularities on their surfaces.
• When they move against each other, these irregularities interlock, producing friction.
• When a lubricant is applied, it forms a thin film between the surfaces.
• This film fills up the irregularities and prevents direct metal-to-metal contact.
• As a result, surfaces slide more smoothly over each other, and friction is greatly reduced.

Examples:

• Oil is used in bicycle chains and motor engines.
• Grease is used in bearings of wheels and axles.
• Graphite powder is used in locks.

Thus, lubricants play a vital role in reducing friction, protecting machine parts and saving energy.

Answer:

Ball bearings:

Ball bearings are small metal balls placed between moving parts of machines, such as between a wheel and its axle.

Working principle:

• Without ball bearings, the wheel would slide over the axle, and there would be large sliding friction.
• When ball bearings are inserted, the inner ring is fixed to the axle and the outer ring is attached to the wheel.
• The balls roll between these two rings.
• Now, instead of sliding friction between axle and wheel, there is mainly rolling friction between the balls and the rings.

How friction is reduced:

• Rolling friction is much smaller than sliding friction.
• Therefore, ball bearings reduce friction significantly and allow smooth and easy rotation of wheels and other parts.

Ball bearings are widely used in bicycle hubs, ceiling fans, motors and various machines to reduce friction and wear.

Answer:

Streamlined shape:

A streamlined body is narrow at the front, broader in the middle and tapering at the back. This shape allows the fluid (air or water) to flow smoothly around the body.

How it reduces friction (drag):

• When objects move through fluids, they experience fluid friction, also called drag.
• If the shape is blunt or irregular, the fluid cannot flow smoothly, and resistance is large.
• A streamlined shape lets air or water flow easily around the object, reducing turbulence and drag.
• As a result, less force is needed to move the object at a given speed.

Application in vehicles and aircraft:

• Cars, buses, trains and aeroplanes are designed with streamlined bodies to reduce air resistance.
• Ships and submarines are given streamlined shapes to move easily through water.
• Reduced drag means higher speeds, lower fuel consumption and smoother motion.

Thus, a streamlined shape helps in reducing fluid friction and improves the efficiency and performance of moving vehicles and aircraft.

Answer:

Fluid friction or drag:

The frictional force exerted by fluids (liquids and gases) on objects moving through them is called fluid friction or drag.

Factors affecting fluid friction:

• Speed of the object: Higher speed ⇒ greater fluid friction.
• Shape and size of the object: Blunt shapes experience more drag; streamlined shapes experience less.
• Nature of the fluid: Thicker (more viscous) fluids like oil offer more friction than thinner fluids like air or water.

Examples from daily life:

• Aeroplanes experience air resistance while flying; their streamlined design helps reduce drag.
• Cyclists lean forward to offer less area to the air and reduce air resistance.
• Boats and ships face water resistance; their design helps them cut through water smoothly.
• A parachute opens to increase air resistance and slow down the falling person.

Thus, fluid friction or drag is important to consider in the design of vehicles, aircraft and many sports to improve performance and safety.

Answer:

Streamlined bodies of birds:

• Birds fly through air, which is a fluid.
• Their bodies are narrow at the front and tapering at the back, making them streamlined.
• This shape allows air to flow smoothly around their bodies, reducing air resistance.

Streamlined bodies of fishes:

• Fishes swim in water, another fluid.
• Their bodies are also streamlined, helping them cut through water with minimum resistance.
• As a result, they can swim rapidly and efficiently with less effort.

Relation with fluid friction:

• When a body moves through a fluid, it experiences fluid friction or drag.
• Streamlined shapes reduce the magnitude of fluid friction acting on the body.
• Therefore, birds and fishes with streamlined bodies can move more easily by overcoming less resistive force.

Hence, streamlined body shapes of birds and fishes are natural adaptations that reduce fluid friction and help in smooth movement through air and water.

Answer:

Difficulty in walking:

• On a wet or oily floor, a thin layer of water or oil covers the surface.
• This layer reduces the friction between the floor and our shoes or feet.
• With less friction, our feet cannot get a firm grip on the floor.
• As a result, our feet tend to slip, and walking becomes difficult and unsafe.

Relation to friction:

• Walking requires sufficient friction between foot and ground to provide grip.
• Wet or oily surfaces are smoother effectively, so friction is reduced.
• Low friction means reduced grip and increased chances of slipping.

Precautions:

• Walk slowly and carefully on wet or oily floors.
• Use footwear with rough or anti-skid soles to increase friction.
• Clean up oil or water spills immediately to restore normal friction.
• Place caution signs in areas where the floor is wet.

Thus, reduced friction on wet or oily floors makes walking difficult, and proper precautions are necessary to avoid accidents.

Answer:

Reason for treads on tyres:

• Tyres are given grooves or treads to increase friction between the tyre and road surface.
• These treads provide better grip, improving safety and control of the vehicle.

Help in dry conditions:

• On a dry road, treads increase the area of contact and provide more edges that grip the road.
• This increased friction prevents the tyre from slipping during acceleration, braking or turning.

Help in rainy conditions:

• When the road is wet, water can form a thin layer between tyre and road, reducing friction.
• Treads allow water to escape through the grooves instead of staying between tyre and road.
• This keeps more rubber in contact with the road, maintaining sufficient friction.

Therefore, treads on tyres improve grip and safety in both dry and wet conditions by maintaining adequate friction with the road surface.

Answer:

Spikes in shoes:

• Athletes wear shoes with metal or rubber spikes on their soles.
• These spikes dig into the ground and increase friction between the shoes and the track.

Relation to friction:

• High friction is needed for quick starts, sudden stops and sharp turns.
• Spikes increase friction, giving the athlete a strong grip.
• This helps in better acceleration, balance and performance.

If smooth-soled shoes are used:

• Friction between the shoes and track would be much less.
• The athlete may slip during start, running or stopping.
• Running speed and stability would decrease, and risk of injury would increase.

Hence, spikes help athletes by increasing friction and providing the necessary grip required for safe and efficient running.

Answer:

(a) New bicycle tyres vs old tyres:

• New tyres have deep treads (grooves) on their surfaces.
• Treads increase friction between tyre and road, giving better grip.
• Old, worn-out tyres have smooth surfaces with faded treads, so friction is less and tyres may skid easily.

(b) Rough-handled tools:

• Tools like screwdrivers and pliers often have rough or patterned handles.
• Roughness increases friction between the tool handle and our hand.
• Higher friction allows us to grip the tool firmly without slipping, making work easier and safer.

(c) Powder on carrom board:

• Carrom powder, usually talcum or boric powder, is sprinkled on the board to make its surface smooth.
• The powder reduces friction between the striker/coins and the board.
• Lower friction helps the coins to slide smoothly over the board with less force.

These examples show how friction is increased or reduced depending on whether strong grip or smooth motion is required.

Answer:

Situation without trolley:

• When we push a heavy box directly on the floor, its entire bottom surface is in contact with the floor.
• Movement involves sliding friction between box and floor.
• Sliding friction is quite large, so a lot of effort is required.

Using a trolley with wheels:

• When the box is placed on a trolley, the trolley’s wheels roll on the floor.
• Now, instead of sliding friction between box and floor, there is mainly rolling friction between wheels and floor.
• Rolling friction is much smaller than sliding friction.

Result:

• Less force is required to move the box when it is on the trolley.
• Movement becomes easier, smoother and faster.

Thus, the use of wheels converts sliding friction into rolling friction, which greatly reduces resistance and makes motion easier.

Answer:

Working of brakes:

• When brakes are applied, brake pads or brake shoes press against the moving wheel or disc.
• Friction between the brake pads and the wheel opposes the motion of the wheel.
• This frictional force converts the kinetic energy of the moving vehicle into heat and slows down the wheel.
• If the brakes continue to be applied, the vehicle eventually comes to a stop.

Why brake pads wear out:

• During braking, brake pads rub continuously against the wheel or disc.
• Friction between the surfaces causes gradual wear, removing small particles of material from the pads.
• Over time, the pads become thinner and less effective at producing friction.
• As a result, braking power decreases and stopping distance increases.

Need for replacement:

• To maintain safe and effective braking, worn-out brake pads must be replaced at regular intervals.
• New pads restore sufficient friction and ensure the vehicle can stop within a safe distance.

So, friction is essential for braking, but the same friction causes wear, making timely replacement of brake pads necessary for safety.

Answer:

Consider the example of a car tyre:

Helpful effects:

• Friction between the tyre and road provides grip required for the car to move forward when the wheels rotate.
• It also allows the car to turn safely without skidding.
• When brakes are applied, friction helps in stopping the car.

Harmful effects:

• Friction between the tyre and road causes the tyre surface to wear out over time.
• It also converts part of the car’s energy into heat, reducing fuel efficiency.
• Excessive friction may lead to overheating and damage to tyres.

Thus, in the same situation, friction helps the car to move and stop safely (useful) but also causes wear and energy loss (harmful). This shows the dual nature of friction as a necessary evil.

Answer:

In writing:

• Friction between pen/pencil and paper helps in leaving marks on the page, allowing students to write notes and solve questions.
• Friction between fingers and pen/pencil helps in gripping it comfortably.

Using books and notebooks:

• Friction between pages of a book keeps them from slipping too easily, so books do not close by themselves.
• Friction between the book and desk prevents the book from sliding off easily.

Classroom furniture:

• Friction between chair legs and floor keeps the chairs steady without sliding when students sit down or get up.
• Friction between the soles of shoes and the floor prevents students from slipping while moving in the classroom.

Thus, friction plays a quiet but important role in many aspects of a student’s study environment, making learning safe and comfortable.

Answer:

Materials needed: A small wooden block, spring balance, three surfaces – glass plate, wooden board and sandpaper sheet.

Procedure:

1. Place the glass plate on the floor. Put the wooden block on it and tie it to a spring balance.
2. Pull the block slowly using the spring balance so that it just starts moving with uniform speed.
3. Note the reading on the spring balance – this gives frictional force on glass.
4. Repeat the same steps with the wooden board and then with the sandpaper sheet as the surface.
5. Record the readings for all three surfaces.

Observation and conclusion:

• The surface which gives the smallest reading on the spring balance offers the least friction.
• The surface with the largest reading offers the most friction.
• We usually find that glass (smooth) offers least friction, wood has moderate friction and sandpaper (rough) offers the most friction.

Thus, by comparing the force needed to move the block on different surfaces, we can conclude which surface has higher or lower friction.

Answer:

Reason for using rollers:

• When two surfaces slide over each other, they experience sliding friction, which is comparatively large.
• To reduce this friction, surfaces are sometimes replaced by rollers or cylinders which roll over each other.
• This changes the motion from sliding to rolling.

Effect on friction and efficiency:

• Rolling friction is much smaller than sliding friction for the same materials.
• Less friction means less heat production and less wear and tear.
• As a result, less energy is needed to move the parts, so the machine becomes more efficient.

Examples:

• Conveyor belts moving on rollers.
• Heavy gates or sliding platforms moving on rollers.

Thus, designing machine parts to move on rollers instead of sliding surfaces reduces friction and improves the efficiency and life of machines.

Answer:

Football:

• Friction between the ball and ground slows the ball and finally brings it to rest, allowing control and passes.
• Friction between players’ shoes and the ground prevents slipping and helps them run, stop and change direction quickly.

Cricket:

• Friction between the ball and the pitch affects how the ball moves or spins after bouncing, making the game interesting.
• Friction between bat handle and hands helps batsmen grip the bat firmly and play powerful shots.

Skating:

• In ice skating, friction between skates and ice is very low, allowing smooth and fast gliding.
• Skaters still need some friction to push against the ice to move forward and to stop.

So, in every sport, friction must be present in the right amount – not too high and not too low – to ensure good performance and safety.

Answer:

Working of a matchstick:

• The head of a matchstick is coated with chemicals that can ignite when heated by friction.
• The side of the matchbox has a rough surface also coated with special chemicals.
• When the match head is quickly rubbed against this rough surface, friction produces heat.
• This heat is sufficient to ignite the chemicals, and the matchstick catches fire.

Why it does not burn on its own:

• When the matchstick is lying still, there is no rubbing and hence no frictional heating.
• The temperature of the match head remains below its ignition temperature.
• So, it does not catch fire by itself while lying in the box.

Therefore, friction is necessary to produce the heat required for lighting a matchstick, but in the absence of frictional rubbing, it remains safe.

Answer:

Reasons for using rubber:

• Rubber provides better friction with surfaces like roads and floors than smooth metals.
• It is flexible and can adjust to small irregularities on the surface, increasing contact area and grip.
• Rubber absorbs shocks and vibrations, making walking or riding comfortable.

Why not metal:

• Metal soles or tyres would be very slippery on smooth or wet surfaces because they provide less friction.
• They would transfer shocks directly to the body and vehicle, making movement uncomfortable and unsafe.

Conclusion:

Even though metal is stronger, rubber is preferred for shoe soles and tyres due to its higher friction, flexibility and shock-absorbing properties, which ensure safety and comfort.

Answer:

Explanation:

• In winter, our hands feel cold due to low temperature.
• When we rub our palms together, the skin of one hand slides over the skin of the other hand.
• Due to friction between the two surfaces, mechanical energy of motion is converted into heat energy.
• This heat warms up the skin and we feel warmer.

Role of friction:

• Friction is responsible for converting kinetic energy into heat during rubbing.
• Without friction, rubbing hands would not produce any noticeable heat.

Thus, friction helps us keep our hands warm in winter by producing heat when we rub them together.

Answer:

Main points about friction:

• Friction is a contact force that opposes relative motion between surfaces.
• It is caused by interlocking of microscopic irregularities on surfaces.
• There are different types: static, sliding, rolling and fluid friction (drag).
• Static friction is usually greater than sliding friction; rolling friction is the least.
• Friction is both useful (walking, writing, driving) and harmful (wear, heating, energy loss).
• Friction can be increased (rough surfaces, treads, chalk) or reduced (lubrication, polishing, ball bearings, streamlining) as needed.
• Fluid friction depends on speed, shape and nature of the fluid. Streamlined shapes reduce drag.

Importance for engineers and designers:

• Engineers must reduce friction in machines to improve efficiency and reduce wear and fuel consumption.
• Designers of vehicles, aircraft and ships need to consider air and water resistance and use streamlined shapes.
• Construction engineers must ensure that floors, roads and tyres provide proper friction for safety.
• Sports equipment, shoes and tools are designed considering the right amount of friction for best performance.

Thus, understanding friction is essential not only for students but also for engineers and designers to create safe, efficient and comfortable systems in everyday life.