FRICTION

Friction Ff is a force that acts between 2 touching objects

• always parallel to the 2 surfaces in contact• always opposite the direction of (attempted) motion

so make sure you make it negative when placed in the Fnet eq’n

3 Types of Friction1. Static - Ffs – opposes the start of motion

For any 2 given surfaces, it has a range of values:

0 < Ffs < max when motion begins

2. Sliding (kinetic) - Ffk – opposes the actual motion• Has a constant value for any 2 given surfaces• Contrary to popular belief, does NOT depend on •Amount of surface area touching• Relative speed between the 2 surfaces

3. Rolling – like with a ball – more in Ch 11

The math of friction: Ff = μFN

so the amount of friction depends on 2 things:

1. μ is the Greek letter mu• it represents the coefficient of friction • “the nature of the 2 surfaces in contact…”• it has no units: μ = Ff / FN would cancel the

only units of Newton/Newton • its value is determined experimentally by

the 2 materials in contact (see chart)Note: static friction is generally greater than

sliding friction for any 2 surfaces. This explains why it’s harder to get an object moving than it is to keep it moving! (Not due to inertia!)

Coefficients of Friction (approximate)

2 Surfaces in Contact μ for static friction (μs)

μ for kinetic friction (μk)

Wood on wood Up to .5 .2Ice on ice Up to .2 .03Lubricated steel on steel

Up to .2 .07

Dry steel on steel Up to .9 .6Rubber on dry concrete

Up to 1.2 .8

Rubber on wet concrete

Up to .8 .5

Rubber on dry asphalt

Up to .7 .5

Rubber on wet asphalt

Up to .75 .25

Teflon on Teflon Up to .04 .04

(The math of friction: Ff = μFN)

(so the amount of friction depends on 2 things:)

2. FN is the normal force • Recall this is the perpendicular supporting

force of a surface that’s under an object• Determined by how much the 2 surfaces

are pressed together as they try to move across each other• So while FN is not the weight, the weight

will often play some role here

This is a magnitude only equation – it only determines the size of the force of friction. Ff’s direction is always negative, but we don’t deal with that when using this equation.

Does Friction only occur between solids? No, also between fluids – anything that flows

liquids and gases are both fluidsEx: Air resistance …which we will get

into in greater detail a little later in this chapter.