Class 9 > Unit # 03: Dynamics > Friction

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Unit No. 3 Dynamics - Friction, Types, and Applications

Define Friction, its cause and explain co-efficient of friction.

Friction

"When a body moves over the surface of another body, an opposing force is produced. This opposing force is called the force of friction."

Explanation

Suppose a wooden block is placed on a table with a spring balance attached to it. If we apply a very small pulling force $F$ gradually, the block does not move until the applied force reaches a critical value. If $F$ is less than this critical value, static friction balances the applied force according to Newton's Third Law. This opposing force is known as friction.

Causes of Friction

Examining material surfaces under a microscope reveals that they are not perfectly smooth; even highly polished surfaces have microscopic irregularities, elevations, and depressions. When one surface rests on another, these microscopic peaks and valleys interlock, opposing relative motion between the surfaces.

Co-Efficient of Friction

Friction is self-adjusting up to a certain maximum limit known as the limiting friction ($F_s$). The limiting friction is directly proportional to the normal reaction force ($R$):

$F_s \propto R$

The ratio between the limiting friction force ($F_s$) and the normal reaction ($R$) is a constant represented by the coefficient of friction ($\mu$):

$\mu = \frac{F_s}{R} \quad \implies \quad F_s = \mu R \quad \text{--- (i)}$

When a body rests horizontally on a surface, its downward weight $W$ is balanced by the normal reaction $R$ ($R = W = mg$). Substituting $R = mg$ into equation (i) gives:

$F_s = \mu m g \quad \text{or} \quad \mu = \frac{F_s}{mg}$

Define types of friction.

• Static Friction: The opposing force acting on an object at rest that resists its tendency to start moving. Maximum static friction is called limiting friction.
• Kinetic Friction: The opposing force that acts on a moving object to resist its ongoing motion. In most practical scenarios, static friction is greater than kinetic friction.
• Sliding Friction: The friction experienced when one body slides directly across the surface of another body.
• Rolling Friction: The resistance encountered when a body rolls over a surface (such as wheels or ball bearings). Rolling friction is significantly smaller than sliding friction.

Write advantages, disadvantages and methods to reduce friction.

Advantages of Friction

• Walking: We could not walk without friction between our shoes and the ground. Pushing backward allows the frictional grip to propel us forward.
• Writing: Holding and writing with a pencil requires friction between the fingers and the pencil, as well as between the graphite tip and paper.
• Fasteners: Nails stay securely embedded in wood and nuts and bolts hold structures together due to frictional grip.

Disadvantages of Friction

• Energy Loss and Fuel Consumption: In vehicles (cars, boats, aeroplanes), excess friction wastes mechanical energy, requiring extra fuel consumption.
• Heat Generation: According to the Law of Conservation of Energy, energy lost to friction is converted into thermal energy (heat), which can overheat machine parts.
• Reduced Efficiency: Friction lowers the mechanical efficiency of machines below 100%.
• Wear and Tear: Continuous friction wears down moving machine components and can pose fire hazards.

Methods of Reducing Friction

• Lubrication: Applying proper lubricants (oil or grease) between moving machine parts creates a thin fluid layer that separates sliding surfaces.
• Using Ball Bearings: Replacing sliding friction with rolling friction by introducing ball bearings or roller bearings in machinery.
• Streamlining: Designing fast-moving objects (cars, aeroplanes, high-speed trains) with smooth, oblong shapes to reduce air resistance (fluid friction).

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