What is friction?

Short Answer:

Friction is a resisting force that acts between two surfaces in contact when one surface moves or tries to move over the other. It always acts in the opposite direction to the motion or tendency of motion. Friction is caused by the microscopic roughness of surfaces and the interlocking of surface irregularities. It plays an important role in everyday life and engineering applications — helping in walking, braking, and gripping — but it can also cause wear and energy loss in machines.

Detailed Explanation :

Friction

In Engineering Mechanics, friction is one of the most important concepts in the study of motion and equilibrium. It is a type of resistive force that develops when two bodies come into contact and one body tends to slide or roll over the other. The direction of friction is always opposite to the applied force or the motion of the body.

Friction is a surface phenomenon that occurs because no surface is perfectly smooth. Even surfaces that appear smooth under normal observation have microscopic irregularities or roughness. When two such surfaces are pressed together, their irregularities interlock, and a force is required to overcome this interlocking. This resisting force is called friction.

Definition

Friction can be defined as:
“The resisting force developed at the surface of contact of two bodies, which opposes the relative motion or the tendency of motion between them.”

It acts tangentially to the surfaces in contact and in the opposite direction to the motion or the applied force.

Mathematically, friction is expressed as:

where,

  • F = Force of friction
  • μ (mu) = Coefficient of friction
  • N = Normal reaction between the surfaces

Causes of Friction

The main causes of friction between two surfaces are:

  1. Surface Roughness:
    Even smooth surfaces have microscopic irregularities that interlock when in contact, producing friction.
  2. Adhesion Between Surfaces:
    When two surfaces come in contact, the molecules at the interface attract each other, causing adhesion that resists motion.
  3. Deformation:
    In soft materials, friction arises due to small deformations at the surface where the materials come into contact.
  4. Interlocking of Surface Irregularities:
    The peaks and valleys of two rough surfaces interlock, creating resistance when one surface moves over the other.

Types of Friction

Friction can be classified into the following main types:

  1. Static Friction:
    • It is the frictional force that acts on a body at rest when an external force is applied, but the body does not move.
    • Static friction prevents the start of motion.
    • It increases with the applied force until it reaches its maximum limit, known as limiting friction.
    • Example: Pushing a heavy box that does not move initially due to resistance.
  2. Kinetic (or Sliding) Friction:
    • It is the frictional force acting on a body that is already in motion.
    • It always acts opposite to the direction of motion.
    • Its magnitude is usually less than limiting friction.
    • Example: Friction between the surfaces of a sliding block and the floor.
  3. Rolling Friction:
    • It occurs when a body rolls over another surface, such as a wheel or a ball bearing.
    • Rolling friction is much smaller than sliding friction because there is less surface contact.
    • Example: The movement of car tires on a road.
  4. Fluid Friction:
    • It is the resistance offered by fluids (liquids or gases) to the motion of objects through them.
    • It depends on the viscosity of the fluid and the speed of the object.
    • Example: Air resistance on a moving car or airplane.

Laws of Friction

The following are the laws of dry friction, proposed by Coulomb:

  1. Law 1:
    Frictional force always acts opposite to the direction of motion or the tendency of motion.
  2. Law 2:
    The magnitude of frictional force is independent of the area of contact between the two surfaces, as long as the normal reaction remains the same.
  3. Law 3:
    The limiting friction is directly proportional to the normal reaction between the surfaces.
  1. Law 4:
    Kinetic (sliding) friction is slightly less than the limiting friction.
  2. Law 5:
    The coefficient of friction depends on the nature and condition of the contacting surfaces, not on their area.

Coefficient of Friction

The coefficient of friction (μ) is defined as the ratio between the limiting friction and the normal reaction between two surfaces:

  • It is a dimensionless quantity.
  • It depends on the nature of the material, surface roughness, and lubrication conditions.
  • Generally, μ < 1 for most practical surfaces.

Typical Values:

  • Steel on steel: 0.15 – 0.6
  • Rubber on concrete: 0.7 – 0.9
  • Ice on ice: 0.03

Advantages of Friction

  1. Helps in Walking:
    Without friction, we could not walk as our feet would slip on the ground.
  2. Enables Braking:
    Brakes in vehicles work due to friction between brake pads and wheels.
  3. Provides Grip:
    Friction helps in holding objects and prevents slipping.
  4. Transmission of Power:
    Belts, pulleys, and clutches depend on friction for power transmission.
  5. Supports Stability:
    Friction keeps objects at rest by resisting external forces.

Disadvantages of Friction

  1. Causes Wear and Tear:
    Continuous friction between moving parts causes material wear.
  2. Energy Loss:
    Energy is wasted in the form of heat due to friction.
  3. Reduces Efficiency:
    Machines require more power to overcome friction.
  4. Generates Noise and Vibration:
    In mechanical systems, friction may produce unwanted noise and vibrations.

Methods to Reduce Friction

Although friction is useful, excessive friction is undesirable. The following methods are used to minimize friction:

  1. Lubrication: Applying oil or grease reduces surface contact and hence friction.
  2. Polishing: Smoothing surfaces reduces surface irregularities.
  3. Use of Ball Bearings: Rolling friction is less than sliding friction; hence, bearings are used.
  4. Use of Streamlined Shapes: Reduces air or fluid resistance.
  5. Proper Material Selection: Using smoother and compatible materials lowers friction.

Practical Applications of Friction

  • Used in brakes and clutches for vehicles.
  • Helps in fixing nails or screws tightly.
  • Essential in belt drives and pulley systems.
  • Used in walking, gripping, and climbing.
  • Required in locomotives and tires for traction.
Conclusion

In conclusion, friction is a resistive force that acts between two contacting surfaces and opposes motion or the tendency of motion. It arises due to surface roughness and molecular adhesion. Friction is both beneficial and undesirable — it enables walking, braking, and gripping but also causes wear and energy loss in machines. By understanding the nature of friction and using techniques like lubrication, engineers can effectively control and utilize friction for desired applications.