6.2 Friction

Friction is the force that opposes motion between surfaces in contact. It's crucial for everyday activities like walking and driving, but can also hinder movement in machines. Understanding friction is key to solving real-world physics problems.

Static friction prevents objects from starting to move, while kinetic friction opposes motion once it begins. These forces depend on surface characteristics, material properties, and environmental factors. Mastering friction concepts helps in analyzing complex mechanical systems and predicting object behavior.

Friction

Principles of friction in physics

• Friction opposes relative motion between two surfaces in contact
  • Acts parallel to surfaces and opposite to direction of motion or attempted motion
  • Converts kinetic energy into thermal energy (heat)
• Caused by interactions between microscopic irregularities (asperities) on surfaces
  • Asperities interlock and resist motion
• Crucial role in everyday life
  • Enables walking, running, and driving without slipping
  • Allows objects to remain stationary on inclined surfaces (ramps, hills)
  • Necessary for operation of mechanical devices (brakes, clutches)

Static vs kinetic friction

• Static friction prevents an object from starting to move when force is applied
  • Acts when there is no relative motion between surfaces
  • Maximum static friction force $f_s$ proportional to normal force $N$ and coefficient of static friction $\mu_s$: $f_s \leq \mu_s N$
  • If applied force is less than maximum static friction force, object will not move
• Kinetic friction opposes motion of an object once it starts moving
  • Acts when there is relative motion between surfaces
  • Kinetic friction force $f_k$ proportional to normal force $N$ and coefficient of kinetic friction $\mu_k$: $f_k = \mu_k N$
  • Coefficient of kinetic friction typically less than coefficient of static friction for same surfaces (rubber on concrete)
• Transition from static to kinetic friction occurs when applied force exceeds maximum static friction force
  • Object starts to move, friction force drops from maximum static to kinetic friction force

Friction in Newtonian mechanics

• Solving problems involving friction using Newton's laws:
  1. Identify surfaces in contact and determine direction of friction force
  2. Draw free-body diagram with normal force, friction force, and other forces acting on object
  3. Apply Newton's second law ($\sum F = ma$) to object, considering force components parallel and perpendicular to surface
  4. Use appropriate friction coefficient (static or kinetic) to relate friction force to normal force
  5. Solve resulting equations for desired quantities (acceleration, maximum angle before sliding)
• Example: Object on an inclined plane (ramp)
  • Normal force $N$ perpendicular to surface, given by $N = mg \cos\theta$ ($\theta$ is angle of inclination)
  • Friction force $f$ acts parallel to surface and opposite to direction of motion (or attempted motion)
  • Component of object's weight parallel to surface is $mg \sin\theta$
  • Apply Newton's second law to determine acceleration or maximum angle of inclination before object starts to slide

Factors affecting friction

• Surface characteristics
  • Roughness of surfaces in contact (affects interlocking of asperities)
  • Contact area between surfaces (surprisingly, friction is generally independent of apparent contact area)
• Material properties
  • Hardness and elasticity of materials (influence deformation of surfaces)
  • Chemical composition (affects adhesion between surfaces)
• Environmental factors
  • Presence of contaminants or lubricants (lubrication can significantly reduce friction)
  • Temperature (can affect material properties and lubricant viscosity)
• Type of motion
  • Sliding friction vs. rolling friction (rolling friction is typically much lower than sliding friction)