Friction

  • Definition of Friction:

    • Friction is a force that resists the movement of two contacting surfaces sliding relative to one another.

    • Acts tangential to the surface at contact points and opposes motion.

  • Dry Friction:

    • Also known as Coulomb friction, named after the French physicist Charles-Augustin de Coulomb, who studied it in 1781.

    • Occurs without lubrication between surfaces.

    • Heat generated by the abrasive action of friction is noticeable when sharpening a metal blade with a grinder.

  • Theory of Dry Friction:

    • Explained by pulling horizontally on a block of uniform weight (W) resting on a rough, non-rigid surface.

  • Equilibrium in Friction Theory:

    • Discusses the effect of distributed normal and frictional loadings indicated by their resultant forces (N and F) in a free-body diagram.

  • Impending Motion:

    • In cases where contacting surfaces are slippery, the frictional force (F) may not balance the pulling force (P), causing the block to tend to slip.

  • Motion:

    • If the magnitude of P exceeds the static frictional force (Fs), the block will start sliding, and the kinetic frictional force (Fk) will be present.

  • Frictional Force Equation:

    • f = μN

      • μs = Static Friction Coefficient

      • μk = Kinetic Friction Coefficient

  • Types of Friction:

    • Static Friction:

      • Acts when the body remains at rest. Its value is between zero and the limiting friction.

    • Limiting Friction:

      • The maximum static frictional force before motion begins.

    • Dynamic Friction:

      • The resistance experienced by a body in motion.

    • Comparison of Static and Kinetic Friction:

      • μs > μk

  • Angle of Friction:

    • The relationship between the frictional force and normal force.

    • tan(θ) = f / N, where θ is the angle of friction.

  • Calculating for Angles of Friction:

    • Given a static friction coefficient μs = 0.04, calculate if a man (75 lb) can stand on a plank (20 lb) without it slipping.

    • Reaction calculations at support points (A and B) yield αA = αB = 21.8°.16

  • Continued Problem Solving:

    • Using equilibrium equations to determine forces acting at points A and B under given conditions.

  • Wedges:

    • A simple machine used to transform an applied force into larger forces directed at right angles to the force.

    • Useful for moving or adjusting heavy loads.

  • Flat Belt Mechanics:

    • When designing drives or brakes for belts, it is essential to determine the frictional forces between the belt and its contacting surface.

  • Flat Belt Forces:

    • Forces in a flat belt system can be represented as:

      • T1 = T2 (Frictionless Condition)

      • T1 ≠ T2 (With Friction Condition)

      • T2 and T1 relations through friction and angle of contact (must be in radians).