Friction – Physics Study Notes

The Origin and Nature of Friction

At the microscopic level, even surfaces that appear smooth to the naked eye are actually quite rough. When two surfaces are placed in contact, the irregularities or “asperities” of one surface interlock with those of the other. This interlocking, combined with molecular-level adhesive forces, creates resistance to motion.

In physics, we categorize friction primarily into two states: Static Friction and Kinetic Friction. Understanding the transition between these two is vital for solving mechanics problems in competitive exams like the IIT JEE. Friction is a non-conservative force, meaning the work done against it is dissipated as heat energy.

Laws of Static Friction

Static friction is the force that prevents two surfaces from sliding past each other when an external force is applied. It is a self-adjusting force; it increases in magnitude to exactly match the external force until it reaches a maximum threshold known as Limiting Friction.

Limiting friction is the maximum value of static friction that comes into play when a body is just on the verge of moving.

The laws governing static friction are:

• The limiting friction is directly proportional to the Normal Reaction (N) between the surfaces.
• It is independent of the area of contact, provided the normal force remains constant.
• The direction of static friction is always opposite to the direction of the applied force that tends to cause motion.
• Mathematically, f_s ≤ μ_sN, where μ_s is the Coefficient of Static Friction.

Kinetic Friction and Its Characteristics

Once the applied force exceeds the limiting friction, the body begins to move, and we enter the regime of Kinetic (or Dynamic) Friction. Unlike static friction, kinetic friction remains relatively constant regardless of the speed of the body over a wide range of velocities.

The magnitude of kinetic friction is given by the formula f_k = μ_kN, where μ_k is the Coefficient of Kinetic Friction. It is an experimental fact that μ_k is generally slightly less than μ_s. This explains why it is harder to start moving a heavy object than it is to keep it in motion.

• Sliding Friction: Occurs when one body slides over another.
• Rolling Friction: Occurs when a body rolls over a surface. Rolling friction is significantly smaller than sliding friction, which is why ball bearings are used in machinery to minimize energy loss.

Friction in Equilibrium Problems

When analyzing a system in equilibrium, you must account for friction as one of the forces in your Free Body Diagram (FBD). If a block is resting on an inclined plane, the component of gravity mg sin θ acts to pull it down the plane, while friction f acts up the plane.

To determine if the block moves, compare the driving force (e.g., mg sin θ) with the limiting friction μ_smg cos θ. If the driving force is less than the limiting friction, the block remains at rest, and the friction force acting on it is exactly equal to the driving force—not the limiting value.

Important Facts / Formulas

Key Points to Remember

• Friction is a contact force; it cannot exist without physical contact between two bodies.
• The coefficient of friction depends only on the nature of the materials in contact, not on the mass or area.
• Always check the “verge of motion” condition by comparing applied force with μ_sN.
• Kinetic friction always opposes the direction of relative velocity between the surfaces.
• If an object is moving at constant velocity, the net force is zero; therefore, applied force equals kinetic friction.
• In problems involving pulleys and strings, friction acts at the interface of the block and the surface.

Quick Revision Summary

• Static Friction: Self-adjusting force that prevents relative motion.
• Limiting Friction: Maximum value of static friction (μ_sN).
• Kinetic Friction: Constant force acting during motion (μ_kN).
• Direction: Always opposes relative motion or tendency of motion.
• Independence: Friction is independent of the actual area of contact.
• Comparison: μ_k is always less than μ_s.
• FBD Strategy: Always resolve forces into components parallel and perpendicular to the contact surface.
• Energy: Work done against friction is dissipated as heat.