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Friction - Methods to Increase and Reduce Friction

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Friction is a contact force that opposes the relative motion between two surfaces in contact. It acts in the direction opposite to the applied force.

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Friction is caused by the interlocking of irregularities on the two surfaces. Even smooth-looking surfaces have microscopic hills and valleys.

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Increasing Friction: Friction can be increased by making surfaces rougher or increasing the force pressing the surfaces together. Examples include treading of tires, using brake pads in bicycles, and kabaddi players rubbing their hands with soil for a better grip.

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Reducing Friction: Friction can be reduced by using lubricants (oil, grease, or graphite), which form a thin layer between surfaces to prevent direct interlocking. Using ball bearings converts sliding friction into rolling friction.

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Rolling friction is significantly smaller than sliding friction, which is why it is easier to roll an object than to slide it. The relationship is expressed as fstatic>fsliding>frollingf_{static} > f_{sliding} > f_{rolling}.

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Fluid Friction (Drag): The frictional force exerted by fluids (gases and liquids) is called drag. It depends on the speed of the object, its shape, and the nature of the fluid. Streamlining (giving a pointed shape) helps reduce drag for airplanes and fish.

📐Formulae

f=μNf = \mu N

fstatic(max)>fsliding>frollingf_{static(max)} > f_{sliding} > f_{rolling}

Force applied−Frictional force=Net Force (m×a)\text{Force applied} - \text{Frictional force} = \text{Net Force (m} \times \text{a)}

💡Examples

Problem 1:

Why do kabaddi players rub their hands with soil before a match?

Solution:

To increase the coefficient of friction (μ\mu).

Explanation:

By rubbing soil on their hands, players increase the roughness of their skin. This leads to better interlocking with the opponent's body, thereby increasing the frictional force and providing a better grip.

Problem 2:

A suitcase is fitted with wheels to make it easier to move. Which type of friction is being utilized here and why?

Solution:

Rolling friction is being utilized.

Explanation:

When an object rolls over a surface, the resistance to its motion is called rolling friction. Since frolling<fslidingf_{rolling} < f_{sliding}, it is much easier to pull a suitcase on wheels than to drag it across the floor.

Problem 3:

Why is it easier to slip on a wet marble floor than a dry one?

Solution:

Water acts as a lubricant reducing the frictional force ff.

Explanation:

Water fills the microscopic irregularities on the marble surface, acting as a thin layer of lubricant. This reduces the interlocking between the feet and the floor, significantly lowering the value of μ\mu, which results in a lack of grip and causes slipping.