FrictionFriction

And Forces in Two DimensionsAnd Forces in Two Dimensions

ReviewReview

A A forceforce is a push or pull. is a push or pull. Newton’s 3 Laws of Motion:Newton’s 3 Laws of Motion:

Objects resist changes in velocity unless Objects resist changes in velocity unless acted upon by a net force.acted upon by a net force.

The magnitude of an object’s The magnitude of an object’s acceleration depends on the net force acceleration depends on the net force and inversely on the object’s mass.and inversely on the object’s mass.

a = Fa = Fnetnet / m / m For every force there is an equal and For every force there is an equal and

opposite counter-force.opposite counter-force.

FrictionFriction

A force that opposes motion.A force that opposes motion. It was once thought that friction was It was once thought that friction was

caused by surface roughness.caused by surface roughness. In this model, microscopic nooks and bumps In this model, microscopic nooks and bumps

on the surface of an object lock and slip on the surface of an object lock and slip against similar features on another surface.against similar features on another surface.

It is now known that this model is It is now known that this model is incorrect.incorrect.

Friction is caused by brief electromagnetic Friction is caused by brief electromagnetic interactions between charged particles in interactions between charged particles in two surfaces.two surfaces.

FrictionFriction Two important types of friction:Two important types of friction:

Static frictionStatic friction Prevents motion from beginningPrevents motion from beginning

Kinetic frictionKinetic friction Prevents motion from continuingPrevents motion from continuing

Static friction is usu. stronger than Static friction is usu. stronger than kinetic friction.kinetic friction.

This is why, when you’re pushing a heavy This is why, when you’re pushing a heavy object across the floor, it’s harder to start it object across the floor, it’s harder to start it moving than it is to keep it moving.moving than it is to keep it moving.

FrictionFriction

FFff = = F FNN FFff = Friction (N) = Friction (N) = Coefficient of friction (no units)= Coefficient of friction (no units) FFNN = Normal force = mg cos = Normal force = mg cos

Coefficient of frictionCoefficient of friction Unique for every pair of substances and Unique for every pair of substances and

set of conditions.set of conditions. Different CoF for static and kinetic Different CoF for static and kinetic

friction.friction.

Coefficients of FrictionCoefficients of FrictionMaterial 1 Material 2

 

Static Sliding  

Aluminum Aluminum 1.05-1.35 1.4  

Aluminum Mild Steel 0.61 0.47  

Cast Iron Cast Iron 1.1 0.15  

Copper Cast Iron 1.05 0.29  

Copper Mild Steel 0.53 0.36  

Copper Steel (304 stainless) 0.23 0.21  

Glass Glass 0.9 - 1.0 0.4  

Glass Nickel 0.78 0.56  

Leather Oak (Parallel grain) 0.61 0.52  

Nickel Nickel 0.7-1.1 0.53  

Oak Oak (parallel grain) 0.62 0.48  

Oak Oak (cross grain) 0.54 0.32  

Rubber Asphalt (Dry)   0.5-0.8  

Rubber Asphalt (Wet)   0.25-0.75  

Rubber Concrete (Dry)   0.6-0.85  

Rubber Concrete (Wet)   0.45-0.75  

Coefficient of FrictionCoefficient of Friction

CoF must be determined CoF must be determined experimentally for any pair of experimentally for any pair of surfaces.surfaces. = F= Fff / F / FNN

Sample ProblemSample Problem The coefficient of static friction The coefficient of static friction

between two blocks of wood is 0.500. between two blocks of wood is 0.500. What applied force is necessary to What applied force is necessary to start a 250.-kg wooden box sliding start a 250.-kg wooden box sliding along a horizontal wooden surface?along a horizontal wooden surface? FFaa must be equal to or greater than F must be equal to or greater than Ff,static.f,static.

FFff = = F FNN FFNN = F = Fww = mg = mg FFNN = (250. kg)(9.81 m/s = (250. kg)(9.81 m/s22) ) FFNN = 2450 N = 2450 N

FFff = (0.500)(2450 N) = (0.500)(2450 N) FFff = 1230 N = 1230 N

Sample Problem 2Sample Problem 2

A box with a mass of 40.0 kg requires an A box with a mass of 40.0 kg requires an applied force of 250. N to keep it moving applied force of 250. N to keep it moving at a constant speed along a horizontal at a constant speed along a horizontal surface. What is the CoF between the box surface. What is the CoF between the box and the floor?and the floor? FFff = = F FNN

FFff = = mg mg (250. N) = (250. N) = (40.0 kg)(9.81 m/s (40.0 kg)(9.81 m/s22)) (250. N) = (250. N) = (392 N) (392 N) = 0.638= 0.638

In lab todayIn lab today

We will calculate the coefficient of friction We will calculate the coefficient of friction between a shoe and the surface of a ramp.between a shoe and the surface of a ramp. You will volunteer the use of your shoes!You will volunteer the use of your shoes! Place the shoes on a ramp.Place the shoes on a ramp. Incline the ramp until the shoe begins to slide.Incline the ramp until the shoe begins to slide. Measure the angle at which the shoe begins to Measure the angle at which the shoe begins to

slide.slide. Calculate the coefficient of friction using some Calculate the coefficient of friction using some

simplifying math.simplifying math. Repeat the procedure but first wet the shoe’s Repeat the procedure but first wet the shoe’s

sole and the board. sole and the board.