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# 8.4: Characteristics of Dry Friction

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### 8.4: Characteristics of Dry Friction

Dry friction occurs when two solid surfaces slide against each other without any lubrication or fluid present. It causes resistance when pushing objects along a surface, like a gardener pushing a wheelbarrow. The force applied to move the cart causes dry friction between the wheel and the ground.

Before the wheelbarrow starts moving, the static frictional force acts tangentially to the contact surface, opposing the force that is about to induce the motion. This frictional force prevents the wheelbarrow from slipping and helps to maintain stability. It is important to note that the static frictional force is only present when there is no relative motion between the two surfaces.

When the wheelbarrow is about to slip, the maximum static frictional force at the interface is directly proportional to the normal force. This relationship is defined by the coefficient of static friction, which depends on the types of materials in contact and the roughness of their surfaces.

Interestingly, the static frictional force is independent of the contact area when the normal pressure is within a specific range that avoids deformation or damage to the contacting surfaces. This means that even if the contact area between the wheel and the ground changes, the static frictional force remains constant as long as the normal pressure stays within this particular range.

Once the wheelbarrow starts slipping and movement occurs, the developed kinetic frictional force comes into play. This force is also proportional to the normal force but is generally lower than the maximum static frictional force. The difference between the two forces can be attributed to the fact that surfaces in relative motion have less time to interlock and create resistance.

However, if the wheelbarrow moves at a very low velocity over the surface, the kinetic frictional force becomes approximately equal to the maximum static frictional force. This is because the slow movement allows for more interaction between the surfaces, leading to increased resistance.

The coefficient of kinetic friction primarily depends on the material's surface properties and the surrounding environment. Factors such as temperature, humidity, and surface contamination can significantly impact the kinetic frictional force.

By examining the factors that influence the coefficients of static and kinetic friction, one can better understand the relationship between the materials in contact, the roughness of their surfaces, and the surrounding environment. This understanding enables individuals to make informed decisions when designing and selecting materials for specific applications, ultimately enhancing the performance and functionality of various systems and devices.