Friction - Class 8 Notes & Olympiad Questions

Friction

a) Friction is essentially a force that comes into play when two surfaces come into contact with each other. It acts as a resistance, opposing the motion that occurs between these surfaces.
b) Friction is a universal phenomenon that occurs whenever surfaces interact and is responsible for various effects such as slowing down moving objects and preventing stationary objects from initiating motion.

Key Points:

a) Friction occurs when two surfaces come into contact: Whenever two surfaces come in contact with each other, friction is likely to arise due to the interaction between their molecules and irregularities on their surfaces.

b) Frictional force opposes the relative motion between the surfaces in contact: Frictional force is the force that resists the motion of one surface over another. It acts in the opposite direction to the intended motion, making it more challenging to move the objects.

c) The force of friction acts opposite to the direction of motion of the object: When an object is moving, the frictional force acts in the direction opposite to the object's motion. This counteracting force tries to bring the object to a halt.

d) Friction slows down moving objects and prevents stationary objects from moving: Friction has a dual role. For moving objects, it acts as a drag force, gradually reducing their speed until they stop. For stationary objects, it acts as a barrier that must be overcome to initiate motion.

e) Friction arises due to irregularities between the surfaces: The irregularities on the surfaces in contact tend to interlock, creating resistance when one surface tries to slide or move over the other. Even seemingly smooth surfaces have microscopic imperfections that contribute to friction.

f) Examples of Friction

1. Walking or Running: When you walk or run, the friction between the soles of your shoes and the ground prevents you from slipping and helps you move forward.
2. Driving a Car: The friction between the tires of a car and the road surface provides the necessary grip for the car to move, turn, and stop.
3. Braking: When you press the brakes in a vehicle, the friction between the brake pads and the wheels generates heat and slows down the vehicle.
4. Writing: When you write with a pen or pencil on paper, the friction between the pen tip and the paper's surface allows the ink to transfer and create marks.
5. Using a Phone or Tablet: The friction between your fingers and the touch screen enables you to control and interact with the device.

Factors Affecting Friction

Friction is influenced by several factors that affect its strength and behaviour. Here are the key factors that influence friction:

1. Nature of Surfaces: The nature of the surfaces in contact plays a significant role in determining friction. Rough surfaces with irregularities tend to create more friction compared to smooth surfaces. Irregularities interlock, causing greater resistance to motion.

2. Surface Area: The contact area between the two surfaces affects friction. Larger contact areas result in more points of interaction and higher friction. For example, pressing a heavy box against the floor increases the contact area and thus the friction.

3. Force Pressing the Surfaces Together: The force pressing the surfaces together, also known as normal force, influences friction. Increasing the normal force increases the frictional force. Heavier objects exert greater normal force, leading to higher friction.

4. Type of Motion: Friction varies based on the type of motion involved. There are three main types of friction: static friction (when an object is at rest), sliding friction (when objects slide over each other), and rolling friction (when one object rolls over another). Sliding friction is generally greater than rolling friction.

5. Temperature: Temperature can affect friction. In some cases, higher temperatures can lead to reduced friction due to the softening of surfaces. On the other hand, increased temperature might increase friction due to the expansion of materials.

6. Lubrication: The presence of lubricants, such as oil, grease, or powder, can significantly affect friction. Lubricants create a thin layer between surfaces, reducing direct contact and interlocking of irregularities. This reduces friction and helps in smooth movement.

7. Smoothness: Smoother surfaces have fewer irregularities, resulting in lower friction. Polished surfaces or those coated with materials like Teflon experience less friction due to reduced interlocking.

8. Velocity: For fluids (liquids and gases), the velocity of the object moving through them affects friction. Higher velocities increase drag force, making it harder to move through the fluid.

9. Pressure: Pressure between surfaces affects friction. Higher pressure increases the number of contact points and interactions, leading to increased friction.

10. Materials Used: Different materials can exhibit different frictional properties.

Types of Friction

Friction comes in different forms depending on the situation and the nature of the surfaces in contact. There are two main types of friction: static friction and kinetic friction.

1. Static Friction

a) Static friction is the type of friction that comes into play when an object is at rest and an external force is applied to set it in motion. It opposes the force trying to initiate motion.
b) The force required to overcome static friction is typically higher than the force needed to keep the object in motion once it has started moving. Static friction prevents objects from sliding or moving when there is no applied force.
c) For example, a book on a table remains stationary until you push it with enough force to overcome static friction.

2. Kinetic Friction

Kinetic friction is the friction that comes into play when objects are already in motion. It includes sliding friction and rolling friction. Kinetic friction opposes the motion of the objects, causing them to slow down over time.

Sliding Friction

a) Sliding friction, also known as kinetic friction, occurs when two surfaces in contact are moving relative to each other. It opposes the motion of the object sliding over another surface.
b) Sliding friction is generally less than static friction, making it easier to keep an object in motion than to start it from rest.
c) For instance, when you push a box across the floor, the sliding friction between the box and the floor slows it down.

Rolling Friction

a) Rolling friction is a type of friction that comes into play when one object rolls over another. It occurs when cylindrical or spherical objects, like wheels or balls, roll on a surface.
b) Rolling friction is generally lower than both static and sliding friction. This is why objects with wheels, like cars and bicycles, can move more efficiently compared to sliding.
c) Rolling friction enables smooth and efficient movement, making it easier to transport heavy loads or move objects over long distances.

Magnitude of Different Types of Friction

Static Friction: Static friction has the highest magnitude among the three types of friction. It acts on stationary objects, resisting any attempt to initiate motion. Static friction increases as the applied force attempting to move the object increases until the maximum static friction force is reached. Once this force is exceeded, the object begins to move, transitioning into either sliding or rolling friction, depending on the situation.

Sliding Friction: Sliding friction, also known as kinetic friction, has a higher magnitude compared to rolling friction. It occurs when two surfaces are in motion relative to each other, and the resistance it creates opposes the object's motion. Sliding friction typically requires more force to maintain the motion of an object compared to rolling friction.

Rolling Friction: Rolling friction generally has the lowest magnitude compared to the other two types of friction. It occurs when an object rolls over another surface, and the interaction between the rolling object and the surface is smoother and involves fewer interlocking irregularities. This allows for more efficient movement compared to the other types of friction.

Advantages and Disadvantages of Friction

Advantages of Friction

1. Walking and Running Without Slipping: Friction between our shoes and the ground provides the necessary grip for us to walk and run safely. Without friction, our shoes would slide on the ground, making it difficult to move and increasing the risk of accidents.

2. Vehicle Movement: Friction between the tires of cars, buses, and other vehicles and the road surface allows them to move forward. Without friction, vehicles would have no traction and wouldn't be able to accelerate, stop, or navigate turns effectively.

3. Lighting a Matchstick: Friction is essential for lighting a matchstick. When you strike a match against the rough surface of the matchbox, friction generates heat, which ignites the chemicals on the match head, leading to a flame.

4. Fixing Nails on Walls: Friction plays a crucial role when we drive nails into walls. The resistance caused by friction prevents the nail from easily slipping out of the wall once it's hammered in, ensuring that objects like picture frames and shelves can be securely hung.

5. Writing on a Blackboard: Friction between the chalk and the blackboard allows us to write. As the chalk moves across the blackboard, it leaves a mark due to the frictional interaction. Without friction, we wouldn't be able to create any visible marks.

6. Creating Heat: Friction generates heat energy. This property is utilized in tools like electric drills, saws, and sandpaper for cutting, shaping, and smoothing various materials.

7. Starting and Stopping Machines: Friction enables machines to start and stop as required. For example, the clutch and brakes in vehicles rely on friction to control the motion of the vehicle.

Disadvantages of Friction

1. Increased Maintenance and Repairs: Friction causes wear and tear on surfaces and components, leading to the need for frequent maintenance and replacement. This can result in higher maintenance costs and downtime for repairs.

2. Heat-Related Issues: Excessive friction can generate heat, causing overheating and damage to components. This is especially problematic in machinery, electronics, and high-speed applications.

3. Loss of Energy: The energy lost due to friction is converted into heat, which can be undesirable in situations where energy conservation is crucial.

4. Reduced Lifespan of Materials: Friction can wear down materials over time, reducing their lifespan. This is evident in everyday items like shoes, where constant friction with the ground leads to deterioration.

5. Noise Generation: Friction can produce noise, which can be bothersome in quiet environments. Squeaks, creaks, and grinding noises are often the result of friction between surfaces.

6. Environmental Impact: The wear particles generated by friction can contribute to environmental pollution, especially in industrial settings.

Ways to Increase and Reduce Friction

Increase Friction

There are methods that can be applied to increase friction when needed to enhance safety, performance, and usability. Some of these ways are:

1. Surface Roughness

a) Increasing the roughness of surfaces can enhance friction by creating more contact points.
b) For example, car tires have tread patterns that improve grip on roads, especially in wet conditions. Hiking shoes also have rugged soles to provide better traction on uneven terrain.

2. Using Coarse Materials

Sandpaper is used to increase friction between surfaces like wood during sanding. A goalkeeper's gloves in soccer are often made with latex to provide a better grip on the ball.

3. Adding Texture

a) Adding texture to surfaces can enhance friction.
b) The dimples on a golf ball are designed to create turbulence in the air, which increases lift and helps control the ball's trajectory. Similarly, the texture on a basketball's surface improves grip for players.

4. Increasing Contact Pressure

a) Applying greater pressure between surfaces increases friction.
b) When a person presses harder on a pen while writing, the increased contact pressure enhances the friction between the pen and paper.

5. Using Adhesive Materials

a) Adhesive materials like tapes or adhesives can increase friction.
b) Velcro is a classic example where hooks and loops create an interlocking effect, increasing friction and holding items together.

6. Applying Pressure

a) Applying pressure between objects increases the friction between them.
b) When a person tightens the cap of a bottle, the pressure between the cap and the neck of the bottle increases, making it harder to open.

7. Creating Interlocking Mechanisms

Mechanisms that create interlocking actions between surfaces increase friction. The teeth of a zipper interlock, allow it to stay closed and prevent unintended opening.

8. Using Grip Enhancers

Chalk is used by gymnasts and weightlifters to absorb moisture from their hands, increasing surface roughness and grip. Climbers often use chalk to maintain a firm grip on rocks.

9. Using Suction Cups

Suction cups create a vacuum between the cup and the surface, increasing friction and holding objects securely. Suction cups are used to attach items like hooks, soap dispensers, and phone holders to smooth surfaces.

Reduce Friction

Friction can be effectively reduced in various situations, leading to improved efficiency, energy savings, and increased lifespan of mechanical components. While friction can be significantly reduced, it's important to note that it can never be completely eliminated due to the presence of surface irregularities at microscopic levels.

Some methods and ways to reduce friction are:

1. Lubrication

a) One of the most effective ways to reduce friction is by using lubricants such as oils, greases, or other specialized substances.
b) Lubricants create a thin layer between two surfaces, minimising direct contact and allowing smoother movement. This reduces the frictional force and helps prevent wear and tear on the surfaces.
c) Lubrication is commonly used in machinery, engines, hinges, and various mechanical systems.

2. Ball Bearings

a) Ball bearings are small metal spheres that are placed between moving parts to facilitate smooth motion. They convert sliding friction into rolling friction by allowing the surfaces to roll over the balls. This reduces the friction and wear on the moving parts.
b) Ball bearings are widely used in machines, vehicles, and industrial equipment.

3. Streamlining

a) Changing the shape of an object to minimize air or fluid resistance is known as streamlining.
b) Streamlined shapes are designed to smoothly move through fluids, reducing drag and friction. This is commonly seen in the design of vehicles, aircraft, and ships to improve efficiency and reduce energy consumption.

4. Using Wheels and Rollers

a) Wheels and rollers are used to replace sliding friction with rolling friction. They reduce the surface area of contact and allow objects to roll instead of slide, which significantly decreases friction.
b) Examples include wheels on vehicles, conveyor belts, and roller skates.

5. Air Cushions

a) Air cushions or air bearings create a layer of air between surfaces, minimizing contact and friction.
b) This is used in applications like hovercrafts, air hockey tables, and air-bearing systems in precision equipment.

6. Polishing and Smoothing Surfaces

Smoothing surfaces through polishing or using coatings can reduce friction by minimizing surface irregularities. However, achieving completely smooth surfaces is challenging due to microscopic imperfections.

7. Using Low-Friction Materials

Using materials like Teflon (PTFE) or other polymers can help reduce friction in specific applications.

8. Reducing Contact Pressure

Lowering the pressure between two surfaces can help reduce friction. This is why roller skates have larger wheels to distribute the weight over a larger area, decreasing the pressure on the ground and reducing friction.

9. Applying Anti-Friction Coatings

Anti-friction coatings, often made of materials like polymers, ceramics, or metals, are applied to surfaces to reduce friction. These coatings provide a barrier that reduces direct contact and minimizes friction.

Fluid Friction

Fluid friction, also known as drag, is a type of resistance that occurs when an object moves through a fluid medium, which includes liquids and gases. It is the force that opposes the motion of the object through the fluid. Fluid friction is a result of the interaction between the object and the molecules of the fluid it is moving through.

Key Points:

1. Opposition to Motion

When an object moves through a fluid, such as air or water, the fluid molecules exert forces on the object in the direction opposite to its motion. This resistance slows down the object and makes it harder to maintain its speed.

Factors Affecting Fluid Friction

1. Speed: Fluid friction increases with the speed of the object. The faster the object moves, the greater the force of fluid friction it experiences.
2. Nature of the Fluid: The viscosity and density of the fluid influence the amount of fluid friction. Thicker and denser fluids result in higher fluid friction.
3. Shape of the Object: The shape of the object moving through the fluid affects the amount of fluid friction. Streamlined shapes, which minimize resistance, experience less fluid friction.

2. Streamlining

To reduce fluid friction and enhance efficiency, objects that move through fluids are often designed with streamlined shapes. Objects with streamlined shapes, which are narrow in front and broader at the back, experience less fluid friction. This shape reduces the resistance of the fluid and allows for smoother movement. This is especially important for vehicles like aeroplanes, submarines, and boats.

3. Applications

a) In air travel, aeroplanes are designed with aerodynamic shapes to reduce air resistance and improve fuel efficiency.
b) Marine vessels like submarines and ships have streamlined designs to minimize water resistance and move more smoothly through the water.
c) Swimmers wear streamlined swimsuits to reduce water resistance and improve their performance.