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 areF = 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?

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

FPerson on

Crate

FEarth on

Crate

F(friction)Groun

d on Crate