Force Of Friction

When an object moves over the surface of another object, a force acts between the two surfaces that opposes the motion. This force causes moving objects like a ball rolling on the ground, a bicycle when pedalling stops, or a car with its engine off, to slow down and eventually come to rest.

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This opposing force is called the force of friction, or simply friction. It acts between the surfaces that are in contact when one surface is moving or attempting to move relative to the other.

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The force of friction always acts in the direction opposite to the direction of motion (or the attempted motion) of the object. For instance, if you push a book to the left, friction acts to the right. If you push it to the right, friction acts to the left. This opposing nature of friction is why moving objects slow down and stop when no external force keeps them moving.

Factors Affecting Friction

The magnitude or strength of the force of friction is not constant; it depends on certain factors related to the surfaces in contact.

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One primary factor is the nature of the surfaces that are in contact. Friction is greater between rough surfaces and smaller between smooth surfaces. Pulling an object like a brick over different types of floors (e.g., bare floor, floor covered with cloth, floor covered with plastic sheet) shows that the force needed to move it varies, indicating different levels of friction. A spring balance is a tool used to measure the force required to overcome friction and start or maintain motion, providing a measure of the friction force.

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Friction also depends on how hard the two surfaces are pressed together. The more strongly the surfaces are pressed against each other, the greater the force of friction. This is why it is harder to drag a heavy object compared to a light one on the same surface, or harder to drag a mat with a person sitting on it compared to an empty mat.

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The fundamental cause of friction is the presence of microscopic irregularities on the surfaces in contact. Even surfaces that appear smooth to the naked eye have tiny bumps and dips. When two surfaces are in contact, these irregularities interlock with each other. To move one surface over the other, a force must be applied to overcome this interlocking. Rougher surfaces have more and larger irregularities, leading to stronger interlocking and thus greater friction. Pressing the surfaces harder increases the degree of interlocking.

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When an object is stationary, the force required to initiate motion and overcome the interlocking irregularities is called static friction. Once the object is moving, the force needed to keep it moving at a constant speed is called sliding friction. Sliding friction is usually slightly less than static friction because the contact points do not get as much time to fully interlock when the object is already in motion. This makes it easier to keep a heavy box moving than to start it moving from rest.

Friction : A Necessary Evil

Friction plays a dual role in our lives: it is both essential (a friend) for many activities and undesirable (a foe) in others.

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Friend (Beneficial Aspects)

Friction is necessary for us to perform many actions:

• Walking: Friction between our shoes and the ground prevents us from slipping, allowing us to walk and run. It is difficult to walk on smooth, wet surfaces due to reduced friction.
• Holding Objects: Friction allows us to grip and hold objects like a glass or a pen. Without friction, objects would simply slip from our hands.
• Writing: Friction between the pen/pencil and paper, or between chalk and a blackboard, is essential for writing.
• Motion of Vehicles: Friction between the tires and the road enables vehicles to start, stop (brakes work using friction), and turn safely.
• Fixing Things: Friction helps in fixing nails into walls or tying knots.
• Construction: Buildings rely on friction between materials to stay standing.

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Foe (Detrimental Aspects)

Friction also has negative consequences:

• Wear and Tear: Friction causes the surfaces in contact to wear down over time. This is seen in worn-out shoe soles, machine parts (like gears or bearings), or steps of stairs that get smooth with use.
• Production of Heat: Friction converts kinetic energy into heat. Rubbing palms together makes them warm. Striking a matchstick generates heat through friction to ignite the chemical. Moving parts in machines get hot due to friction, leading to energy loss (waste of energy).

Increasing And Reducing Friction

Depending on the application, we can deliberately increase or decrease the force of friction.

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Increasing Friction

Friction is increased when a stronger grip or better traction is needed. This is achieved by making the surfaces rougher:

• The soles of shoes and the treads on tires of cars, trucks, and bulldozers are grooved. These patterns increase friction with the surface, providing better grip and preventing slipping.
• Athletes like kabaddi players rub soil on their hands, and gymnasts apply coarse substances. This increases the friction between their hands and opponents or equipment, allowing for a better grip.

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Reducing Friction

Friction is reduced when smooth movement is desired or to minimise wear and energy loss in machines. This is done by:

• Using lubricants: Substances like oil, grease, or graphite are applied between moving parts. Lubricants form a thin layer that prevents the direct contact and interlocking of the irregularities of the surfaces, allowing them to slide or roll more smoothly. Examples include oiling door hinges or using grease in bicycle and motor mechanics.
• Sprinkling fine powder (like talcum powder) on a carrom board reduces friction between the board and the strikers/coins.
• In some advanced applications, an air cushion can be used between moving parts to drastically reduce friction.

Although friction can be significantly reduced, it can never be completely eliminated because all surfaces, even seemingly smooth ones, retain some level of microscopic irregularities.

Wheels Reduce Friction

It is observed that moving an object by rolling it is much easier than sliding it. For example, pushing a heavy book placed on cylindrical pencils or using luggage with rollers requires much less effort than dragging them directly on the surface.

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The resistance to motion experienced when one body rolls over the surface of another is called rolling friction. The resistance experienced when one body slides over another is called sliding friction. Rolling friction is always smaller than sliding friction.

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This fundamental difference explains why the invention of the wheel is considered one of the greatest achievements of mankind. Wheels convert sliding motion into rolling motion, thereby drastically reducing friction and making transportation and movement of heavy objects much more efficient.

Many machines also utilise this principle by replacing sliding parts with rolling elements to reduce friction. Ball bearings are commonly used for this purpose. They consist of small metal balls placed between the moving parts (like between the hub and axle of a wheel or in a ceiling fan). The balls roll, converting the sliding friction between the parts into the much smaller rolling friction.

Fluid Friction

Substances that can flow are called fluids. This category includes both liquids (like water) and gases (like air). Objects moving through fluids also experience a frictional force.

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Fluids exert a frictional force on objects moving through them. This force opposes the object's motion. The frictional force exerted by fluids is often specifically called drag.

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The magnitude of the drag force depends on several factors:

• The speed of the object relative to the fluid.
• The shape of the object.
• The nature of the fluid (e.g., its density and viscosity).

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When objects move through fluids, they must overcome this friction, which consumes energy. To minimise this energy loss, especially for vehicles and aircraft that move at high speeds, their shapes are designed to reduce fluid friction. These shapes are called streamlined shapes.

Nature provides excellent examples of streamlined shapes that have evolved for efficient movement through fluids. Birds and fish have body shapes that minimise friction as they move through air and water, respectively. Engineers and scientists often take inspiration from these natural designs. Aeroplanes, boats, and cars are all designed with streamlined shapes to reduce the drag force exerted by air or water, allowing them to move more efficiently.

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