FRICTION Is F = μN

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”)

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

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

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

Table of μ

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.

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.

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?  Σ F x = F PonC + F fGonC  Σ F y = F GonC + F EonC  a x = [F PonC + μ *F GonC ]/m  a x = [F PonC + μ mg]/m  F PonC = ma x + μ mg  F PonC = (10kg*0.5 m/s²) + (-0.35*10kg*10N/kg)  F PonC = 40 N F Ground on Crate F Person on Crate F Earth on Crate F(friction) Ground on Crate