8.2 : Static Friction

Static friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact. It plays a crucial role in our daily lives, from walking on the ground to driving a car.

For example, consider a scenario where a truck is connected to a car by a rope, ready to tow it along a road. When no external force is applied by the truck, the car remains stationary and is said to be in static equilibrium. In this case, the forces acting on the car, such as gravity and the normal force from the road, are balanced.

When the truck applies the to move the car, the frictional force between the car's tires and the road surface increases linearly, keeping the car in equilibrium.

As the applied force increases, there comes a point when the car slips and starts to move. At this moment, the frictional force between the car and the road decreases slightly due to the transition from static friction to kinetic friction.

Once the car has slipped, the frictional force remains relatively constant. However, as the applied force from the truck continues to increase, the frictional force opposing the car's motion eventually starts to decrease.

The range of static friction where the car remains stationary and in equilibrium, despite increasing applied force is known as the region of impending motion. In this region, the static frictional force is less than the normal force exerted on the car by the road surface.

When the car is on the verge of slipping, the maximum value of static frictional force, called the limiting static frictional force, comes into play. It is proportional to the normal force acting on the car, with a coefficient of static friction that depends on the roughness of the contacting surfaces (in this case, the car's tires and the road).

As the car approaches the point of slipping, the angle formed by the resultant reaction force and the normal force is referred to as the angle of static friction. This angle measures how close the car is to sliding and can be used to determine the effectiveness of the static friction between the car and the road.