NCERT Textbook Questions
(a) Friction opposes the motion between the surfaces in contact with each other.
(b) Friction depends on the nature of surfaces.
(c) Friction produces heat.
(d) Sprinkling of powder on the carrom board reduces friction.
(e) Sliding friction is less than the static friction.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
Correct Answer: (c) static, sliding, rolling
Reason: Static friction is the strongest, followed by sliding friction, and rolling friction is the weakest.
(a) wet marble floor, dry marble floor, newspaper, towel
(b) newspaper, towel, dry marble floor, wet marble floor
(c) towel, newspaper, dry marble floor, wet marble floor
(d) wet marble floor, dry marble floor, towel, newspaper
Correct Answer: (a) wet marble floor, dry marble floor, newspaper, towel
Friction always opposes motion. Since the book is sliding downwards due to gravity, the frictional force will act in the upward direction along the incline of the desk.
It would make it more difficult to walk. Walking requires friction between our feet and the ground so that we can push the ground backward to move forward. Soapy water fills the irregularities of the floor and feet, drastically reducing the friction. This increases the chances of slipping.
Sportsmen use shoes with spikes to increase friction between the shoes and the ground. This gives them a better grip, prevents slipping while running or playing, and helps them to stop or change direction quickly without falling.
Seema will have to apply a larger force. Friction depends on the force pressing the two surfaces together (weight). The heavier box presses harder against the floor, causing more interlocking of surface irregularities. Therefore, the friction is greater for the heavier box, requiring more force to move it.
Friction is caused by the interlocking of irregularities on the two surfaces. In static friction (when object is at rest), the irregularities have enough time to lock into each other tightly. In sliding friction (when object is moving), the contact points on one surface do not get enough time to lock into the contact points of the other surface. So, the interlocking is less strong, making sliding friction less than static friction.
Friction as a Friend (Advantages):
- It allows us to walk without slipping.
- It enables us to write with a pen or chalk.
- It helps to light a matchstick.
Friction as a Foe (Disadvantages):
- It causes wear and tear of machine parts and shoe soles.
- It produces unwanted heat in machines, reducing efficiency.
- It slows down moving objects, requiring more energy to maintain speed.
When objects move through fluids (liquids and gases), they experience an opposing force called **fluid friction** or **drag**. To minimize this friction and save energy, objects are given special shapes called **streamlined shapes**. These shapes (like that of birds, fish, and airplanes) allow the fluid to flow smoothly around the object, reducing the drag.
Extra Important Questions
A spring balance is a device used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.
Detailed Chapter Discussion: Friction
1. Introduction to Friction
You might have seen a driver of a car or a truck slowing down the vehicle at a traffic signal. You, too, slow down your bicycle whenever needed by applying brakes. Have you ever thought why a vehicle slows down when brakes are applied?
Not only vehicles, any object, moving over the surface of another object slows down when no external force is applied on it. Finally it stops. Why do we slip when we step on a banana peel? Why is it difficult to walk on a smooth and wet floor? The answer is Friction.
Definition: Friction is the force that resists the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. It always opposes the relative motion between two surfaces.
2. Factors Affecting Friction
Friction is caused by the irregularities on the two surfaces in contact. Even surfaces which appear very smooth have a large number of minute irregularities on them. When we attempt to move any surface, we have to apply a force to overcome this interlocking.
Friction depends on two factors:
- Nature of Surfaces: Friction is greater if the surfaces are rough (more irregularities) and less if the surfaces are smooth (less irregularities).
- Force Pressing the Surfaces Together: Friction increases if the two surfaces are pressed harder against each other. For example, it is easier to drag a mat when nobody is sitting on it than when a person is sitting on it.
3. Types of Friction
Friction can be classified into different types based on the state of motion:
A. Static Friction
The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. This is the maximum value of frictional force before motion begins.
B. Sliding Friction
The force required to keep the object moving with the same speed is a measure of sliding friction. Sliding friction is slightly smaller than static friction because the irregularities do not get enough time to lock into each other completely when the object is moving.
C. Rolling Friction
When one body rolls over the surface of another body, the resistance to its motion is called rolling friction. Rolling reduces friction. It is always easier to roll than to slide a body over another. That is why luggage is fitted with wheels (rollers).
Comparison: Static Friction > Sliding Friction > Rolling Friction
4. Friction: A Necessary Evil
Friction plays a dual role in our lives. It is both necessary and harmful.
A. Advantages (Why it is necessary)
- Walking: We are able to walk because of friction between our feet and the ground. Without it, we would slip.
- Writing: We can write with a pen or pencil because friction rubs off a small amount of ink or graphite.
- Holding Objects: Friction allows us to hold a glass, a book, or any object.
- Brakes: Interaction between brake pads and wheels stops vehicles.
- Lighting a Match: Friction between the matchstick and the side of the box generates enough heat to light it.
B. Disadvantages (Why it is an evil)
- Wear and Tear: It causes soles of shoes, tyres of vehicles, and machine parts to wear out.
- Heat Generation: Friction produces heat, which can damage machine parts and waste energy.
- Energy Loss: A significant amount of energy in machines is wasted in overcoming friction.
5. Increasing and Reducing Friction
Depending on the situation, we sometimes need to increase friction and sometimes reduce it.
Increasing Friction
- Grooved Soles: Shoes have grooved soles to provide better grip.
- Treaded Tyres: Tyres of cars, trucks and bulldozers are treaded to increase grip with the road.
- Brake Pads: Using brake pads in the brake system of bicycles and automobiles increases friction when needed.
- Gymnasts: They apply some coarse substance on their hands to increase friction for better grip.
Reducing Friction
- Lubricants: Substances like oil, grease, or graphite powder are applied between moving parts of machines. They form a thin layer, preventing direct contact between surfaces and reducing friction.
- Ball Bearings: Since rolling friction is less than sliding friction, sliding is replaced by rolling in most machines by using ball bearings (e.g., in fans, bicycle axles).
- Polishing: Polishing surfaces makes them smoother, reducing irregularities and thus friction.
- Air Cushion: Hovercrafts move on a cushion of air to minimize friction with water.
6. Fluid Friction (Drag)
Air and water (and other fluids) also exert fictional force on objects moving through them. The frictional force exerted by fluids is also called drag.
The frictional force on an object in a fluid depends on:
- Its speed with respect to the fluid.
- Shape of the object.
- Nature of the fluid.
Streamlining: To reduce fluid friction, objects are given special shapes called streamlined shapes. For example, birds and fishes have natural streamlined bodies to move easily. Humans design airplanes, rockets, and ships with similar shapes to minimize energy loss.
50 Key Facts: Friction
Friction is a force that opposes the relative motion between two surfaces in contact. It acts in the direction opposite to the direction of motion.
Friction is caused by the interlocking of irregularities (roughness) on the two surfaces in contact.
Friction depends on the nature of surfaces in contact. Rough surfaces have greater friction, while smooth surfaces have less friction.
Friction depends on how hard the two surfaces are pressed together. Greater the weight (normal force), greater is the friction.
Static friction is the force that opposes the initiation of motion. It comes into play when we try to move an object at rest. It is a self-adjusting force.
The maximum value of static friction just before the object starts moving is called limiting friction.
Sliding friction is the force opposing motion when one body slides over another. It is slightly smaller than static friction.
Rolling friction is the resistance to motion when a body rolls over another. It is much smaller than sliding friction.
Friction produces heat. This is observed when we rub our hands together or when a machine gets hot after running for sometime.
Friction is called a necessary evil because it is essential for many activities (walking, writing) but also causes energy loss and wear & tear.
Walking is possible due to friction between our feet and the ground. We push the ground backward, and friction pushes us forward.
We can write with a pen or pencil because friction rubs off ink or graphite particles onto the paper.
Automobiles stop when brakes are applied due to the friction between the brake linings and the drum of the wheel.
Friction causes wear and tear of shoe soles, tyres, and machine parts over time, which is a disadvantage.
Substances like oil, grease, or graphite that are used to reduce friction are called lubricants. They fill the irregularities of surfaces.
Ball bearings convert sliding friction into rolling friction, significantly reducing energy loss in machines like fans and bicycle hubs.
Fluids (liquids and gases) exert a force of friction on objects moving through them. This frictional force is also called drag.
Giving objects a special shape (streamlined) helps reduce fluid friction. Examples include the shape of boats, airplanes, and fish.
Sportsmen use shoes with spikes to increase friction and get a better grip on the ground to prevent slipping.
Tyres of vehicles are treaded to provide better grip with the road by increasing friction, preventing skids.
A matchstick lights up when rubbed against a rough surface because the heat generated by friction ignites the chemicals.
Gymnasts apply coarse powder on their hands to increase friction for a better grip on bars.
We sprinkle fine powder on a carrom board to reduce friction, allowing the striker to move smoothly.
When door hinges squeak, we add oil drops to reduce friction and allow smooth movement.
A hovercraft creates a cushion of air between its hull and the water/ground to drastically reduce friction.
Fluid friction depends on speed of object, shape of object, and nature of the fluid.
Microscopic hills and valleys on surfaces lock into each other, causing friction.
A spring balance measures the force required to pull an object, giving a direct measure of friction.
Water on a floor reduces friction by filling irregularities. This is why it is easy to slip on a wet floor.
Stepping on a banana peel reduces friction suddenly between the foot and ground, causing a person to slip.
Meteors burn up upon entering the atmosphere due to the extreme heat generated by air friction (drag).
Polishing a surface removes some irregularities, making it smoother and reducing friction.
Rollers are attached to heavy luggage to convert sliding friction to rolling friction, making it easier to pull.
Viscosity is the internal friction of a fluid. Honey has high viscosity, while water has low viscosity.
Friction reduces the efficiency of machines because part of the energy input is lost as heat.
If an object moves to the right, friction acts to the left. It always opposes the relative motion.
Players rub their hands with soil for a better grip of their opponents by increasing friction.
The screeching sound of tires or a violin string is produced due to friction (stick-slip motion).
Friction is a contact force because it requires physical contact between surfaces to act.
Air resistance is a form of fluid friction that slows down objects moving through air, like parachutes.
Maglev trains float above tracks using magnetic levitation, eliminating rolling friction and allowing very high speeds.
Static friction is greater than sliding friction because bonds between surfaces have time to form when at rest.
Sliding friction is much greater than rolling friction, which is why wheels are a revolutionary invention.
We can tie a knot because friction holds the threads together. Without friction, knots would slip open.
Sand is thrown on icy roads to increase friction and improve traction for vehicles.
Friction allows conveyor belts to move items without them slipping off.
It is practically impossible to eliminate friction completely. Even in space, there is some drag from particles.
Ideally, a ladder leaning against a wall needs friction at the ground to prevent it from sliding out.
Flour mills use rough friction stones to grind grain into flour.
Reducing friction conserves energy, which is a major goal in engineering design.