What is Static Friction | Civil Engineering Optional Notes for UPSC PDF Download

Static Friction

What is Static Friction?

Static friction is a force that keeps an object at rest. It can be defined as the friction experienced when trying to move a stationary object on a surface, without actually causing any relative motion between the object and the surface.

In more detail, static friction is the force that exactly balances the applied force, maintaining the object in a stationary state. This force is self-regulating, meaning that it will always be equal and opposite to the applied force, preventing motion.

For example, if an external force F is applied to a stationary object with weight W, the reaction force R due to the weight and the frictional force $f\_r$f_r will act in the opposite direction. When no motion occurs, the external force F and the frictional force $f\_r$fr are equal and opposite, so $F=$−f_r.

Static Friction Examples

• Papers on a tabletop
• A towel hanging on a rack
• A bookmark in a book
• A car parked on a hill

Laws of Static Friction

1. Independence from Area of Contact: The maximum force of static friction does not depend on the contact area between the surfaces.
2. Proportional to Normal Force: The maximum force of static friction is proportional to the normal force. As the normal force increases, the maximum external force the object can withstand without moving also increases.

Limiting Friction

Limiting friction is the maximum value of static friction that occurs just before an object begins to slide over another surface. When an external force exceeds the limiting friction, the object starts to move.

Once motion begins, static friction is no longer applicable, and kinetic friction comes into play.

Laws of Limiting Friction

1. Opposition to Motion: The direction of the limiting frictional force is always opposite to the direction of motion.
2. Tangential Action: Limiting friction acts tangentially to the two interacting surfaces.
3. Proportionality to Normal Reaction: The magnitude of limiting friction is proportional to the normal reaction force between the two surfaces.
4. Dependence on Material and Surface Characteristics: Limiting friction depends on the material and nature of the interacting surfaces and their smoothness.
5. Independence from Shape and Area: For any two given surfaces, as long as the normal reaction is the same, the magnitude of limiting friction is independent of the shape or area of the surfaces in contact.