FRICTIONSLEDS, SANDPAPER, AND LOTS

of SLIDING

Friction

• Any force that resists motion

• It involves objects that are in contact with each other.

• This is the force that keeps an object from sliding down and incline plane.

• Some scientists believe that friction is caused by uneven surfaces of the touching objects – when rubbed together resistance is offered.

• Experiments have shown that tiny particles are actually torn from one surface and imbedded in the other.

• If two surfaces were carefully polished, there is a limit to the amount by which friction may be reduced. If made too smooth, the friction between them actually increases.

Examples of FrictionExamples of Friction

• Tires on the road

• Walking/Running

• Nails into wood

• Sled on hill

Principles of Friction

• Friction acts parallel to the surface that are in contact.

• The direction that friction acts is OPPOSITE the direction of the motion (or intended motion).

Fweight

Fnormal

FappliedFfriction(k)

There are two types of friction:Static Friction – the frictional force present

just before motion begins. Starting Friction is when Static Friction is at is maximum.

Kinetic Friction – the frictional force present with motion

Note: Static Friction is usually higher than Kinetic Friction

• Friction depends on the nature of the material in contact and the smoothness of their surfaces.

• Static Friction is usually higher than Kinetic Friction because it is harder to get something started than it is to keep it going.

• Friction is practically independent of the area of contact

The Coefficient of Friction Frictional Force, both static and kinetic, is directly

proportional to the force pressing the two surfaces together. The more weight, the more friction.

normal

(kinetic)friction kinetic

normal

(static)friction static

normalfriction

F

F

F

F

FF

normalfriction FF The missing link to the above equation is the

Coefficient of Friction, or μ:

PROBLEMS

A 75 kg crate is to be pushed up an incline plane 5 m long that makes an angle of 20° with the horizontal. If the coefficient of static friction between the crate and the inclined plane is 0.20, how much force must be given to get it started up the incline? If the coefficient of kinetic friction is 0.15, how much applied force is needed to keep it going at a constant speed up the incline.

Given: m=75 kg; l=5 m; θ=20°; μs=0.20; μk=0.15

Fw=mgFf

Fn=mg(cosθ) Fa

θ

Fp=mg(sinθ)

)(sin

)(cos

mgF

mgFFFFF

p

snssfpsfa

N 251.38

N 13.138

needed is forcemuch thisLEAST AT d. threshhol theis this:note*

N 51.389aF

)(sin

)(cos

mgF

mgFFFFF

p

snksfpkfa

N 251.38

N 60.103

speed. CONSTANTfor is This :note*

N 98.354aF

Take into account the last problem. After you have the crate moving up the incline at a constant speed, you want to give it an acceleration of 1.7 m/s2. What is the new force needed to accomplish this task?

)(sin

)(cos

mgF

mgFFmaFFF

p

snksfpkfa

N 251.38

N 60.103

N 48.482aF

maFFF pkfa

A 300 kg sled is pulled at constant speed over a level, horizontal, snow covered surface. The rope that is used to pull the sled makes a 30° angle with the horizontal. If the coefficient of friction is 0.10, find the force required.

Given: m=300 kg; θ=30°; μk=0.10

Fn

Fx (cos θ)

Ff

Fw

Fa

θ

Fy (sin θ)

Fax-Ff=max *note: ax is 0 because we are at a constant speed

Fay+Fn-Fw=may *note: ay is 0 because the sled never moves up and down