8.3 : Kinetic Friction

Consider a truck trying to pull a stationary car. As the truck exerts a force on the car, static friction is created at the point of contact between the two surfaces. This frictional force resists the car's movement and keeps it at rest. However, when the applied force by the truck surpasses the limiting static frictional force, an interesting phenomenon occurs. The frictional force at the interface reduces to a lower value, known as the kinetic frictional force. At this point, the car begins to slide and its speed increases.

Kinetic friction, also referred to as dynamic friction or sliding friction, is the force that opposes the relative motion between two objects in contact while they are moving. It acts parallel to the contact surface and in the opposite direction of the motion. In the example of truck and car, kinetic friction is responsible for resisting the car's motion as it starts to slide.

The magnitude of the kinetic frictional force is directly proportional to the normal force acting on the objects with a proportionality constant, denoted as the coefficient of kinetic friction, which is a dimensionless quantity that depends on the properties and condition of the surfaces in contact.

As the car starts to slide with increasing speed, the interface between the truck and the car undergoes a continuous breakdown. This process is responsible for generating kinetic friction, which opposes the car's motion. Interestingly, the magnitude of kinetic friction does not remain constant as the car's velocity increases. Initially, kinetic friction decreases slowly, but it experiences a more significant drop at higher velocities.

The angle between the kinetic friction force and the resultant normal force is called the angle of kinetic friction. This angle is always less than or equal to the maximum static friction angle. Notably, the angle of kinetic friction depends on the materials and conditions of the surfaces in contact.

Various factors, including the condition of the contacting surfaces and the relative velocity between them, influence the coefficient of kinetic friction. Rougher surfaces tend to have higher coefficients of kinetic friction, while smoother surfaces exhibit lower values. Also, lubricants or contaminants can alter the coefficient of kinetic friction by changing the surface conditions.