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Friction

Friction. Is F = μ N. What is Friction?. Resistive force caused by two surfaces rubbing against each other R esists an object's motion - acts opposite direction of motion or "intended motion "

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Friction

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  1. Friction Is F = μN

  2. What is Friction? • Resistive force caused by two surfaces rubbing against each other • Resists an object's motion - acts opposite direction of motion or "intended motion" • Depends on materials, texture of surface (smooth or rough) caused by microscopic or sometimes macroscopic irregularities in surface (“bumpiness”)

  3. Types of Friction · Kinetic: two surfaces sliding against each other, object sliding against a surface (ex: sledding, skiing, snowangels, car brakes, slide at playground, rubbing hands together) · Static: friction between a surface and a non-moving object

  4. Sliding Friction Static Friction • When an object is at rest and another object is attempting to push/pull it across a surface • Gradually increases to a maximum value, peak static friction (Fs,max) Kinetic Friction (Fk) • When an object is already in motion (sliding across a surface) • Generally a constant value, Fk Fs,max > Fk

  5. What affects friction? • Coefficient of Friction (Greek letter μ “mu”) is a measurement of how tacky or smooth 2 surfaces are • units: unitless, just a number • the greater the magnitude, the tackier or rougher the surfaces are F = μN

  6. Table of μ

  7. Try It Yourself! 2. A 25 kg chair initially at rest on a horizontal floor requires a 165 N horizontal force exerted by a person to set it in motion. a. Find the coefficient of static friction between the chair and the floor.

  8. Solution 3. Once the chair is in motion, a 127 N horizontal force exerted by the person keeps it moving at a constant velocity. a. Find the coefficient of kinetic friction between the chair and the floor.

  9. FD Example • A 10 kg crate is being pulled by a person across the factory floor with an acceleration of 0.5 m/s². If the coefficient of kinetic friction between the crate and the floor is 0.35, how much does force does the person have to exert on the crate? • ΣFx = FPonC + FfGonC • ΣFy = FGonC + FEonC • ax = [FPonC + μ*FGonC]/m • ax = [FPonC + μmg]/m • FPonC = max + μmg • FPonC = (10kg*0.5 m/s²) + (-0.35*10kg*10N/kg) • FPonC = 40 N FGround on Crate F(friction)Ground on Crate FPerson on Crate FEarth on Crate

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