Types of Friction>Kinetic Friction

Kinetic friction is the force of resistance that acts when one body slides over another surface. Once an object starts moving, the friction force decreases compared to what was needed to initiate the motion. The force required to maintain steady sliding motion is less than the force required to overcome static friction and start the motion.

Friction between two surfaces in contact when one body is actually sliding over the other body is called kinetic friction or dynamic friction.

or

The force of friction that comes into play when a body is in a steady state of motion over another surface is called the force of kinetic friction.

Fₖ = μₖ N

Where:

• Fₖ = Force of kinetic friction
• μₖ = Coefficient of kinetic friction (constant of proportionality)
• N = Normal reaction between the two surfaces in contact

μₖ = Fₖ/N

The coefficient of kinetic friction is defined as the ratio of force of kinetic friction to the normal reaction between the two surfaces in contact.

• The force of kinetic friction is directly proportional to the normal reaction between two surfaces in contact
• Kinetic friction is independent of the shape and apparent area of the surfaces in contact
• It depends upon the nature and material of the surfaces in contact
• The magnitude of kinetic friction is independent of the relative velocity between the object and the surface (provided the velocity is neither too large nor too small)
• Kinetic friction acts in the direction opposite to the relative motion between surfaces

First Law:
The force of kinetic friction (Fₖ) is directly proportional to the normal reaction between two surfaces in contact.

• Fₖ ∝ N
• Fₖ = μₖ N

Second Law:
The coefficient of kinetic friction is defined as the ratio of the force of kinetic friction to the normal reaction between the two surfaces in contact.

• μₖ = Fₖ/N

Third Law:
The force of kinetic friction is independent of shape and apparent area of the surfaces in contact.

Fourth Law:
The force of kinetic friction depends upon the nature and material of the surfaces in contact.

Fifth Law:
The magnitude of the force of kinetic friction is independent of the relative velocity between the object and the surface (provided that the relative velocity is neither too large nor too small).

When a body slides across a surface, kinetic friction opposes its motion. At the microscopic level, even smooth surfaces have irregularities. As one surface slides over another, these irregularities interlock and create resistance. This resistance is the kinetic friction force.

Key Points to Remember

• Kinetic friction acts only when there is relative motion between two surfaces
• Direction: Kinetic friction always acts opposite to the direction of relative motion
• Magnitude: The magnitude depends only on the normal force and the nature of the surfaces, not on the area of contact or the speed of sliding
• Coefficient remains constant for a given pair of materials (under normal conditions)

Why Kinetic Friction is Less than Static Friction

Once sliding begins, the surfaces are in continuous contact at different points. The deformation of surface irregularities is less severe than when initiating motion from rest. This results in a lower kinetic friction coefficient (μₖ) compared to the static friction coefficient (μₛ), meaning less force is needed to maintain motion than to start motion.

• Kinetic friction is essential in understanding the motion of objects on surfaces
• It helps in calculating the deceleration of moving objects
• Important for designing braking systems in vehicles
• Used in industrial processes involving sliding and movement
• Understanding kinetic friction helps in friction reduction and lubrication techniques
• Critical for safety analysis in transportation and machinery
• Helps predict energy loss due to friction in mechanical systems