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Assignment: Problems p 105 – 107 #’s 15-17 and 19-21. Friction a force that opposes motion What do you think friction depends on? It depends on the two surfaces that are in contact. Not all surfaces have the same amount of friction. Do rough / smooth surfaces have more friction?

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Chapter 5 4111521

Assignment:

  • Problems p 105 – 107

  • #’s 15-17 and 19-21

Friction a force that opposes motion

What do you think friction depends on?

It depends on the two surfaces that are in contact.

Not all surfaces have the same amount of friction.

Do rough / smooth surfaces have more friction?

A rough surface will have less overall contact with an object.

More or less friction?

Smooth surfaces will have more contact with an object.

More or less friction?

It depends on the size of the object.

Does more mass mean more friction?

Does more surfacearea (in contact) mean more friction?

Chapter 5


Friction

Friction...

  • Friction is caused by the “microscopic” interactions between the two surfaces. Your book refers to it as “electromagnetic bonding”


Chapter 5 4111521

Friction is not dependant of surface area in contact, or surface smoothness.

Friction depends on the two types of surfaces in contact.

“Sticky” surfaces “slippery” surfaces

rubber tire and asphalt rubber tire and wet asphalt

rubber and wooden flooring leather (shoes) and wooden flooring

glass and glass paper and a desktop

FRICTONAL COEFFICIENT ( µ )

(mu, “mew” 12th character of the ancient Greek alphabet)

A number (coefficient) that is given to two materials that helps

determine the frictional force between them.

µ will always be less than 1 (one) for us in this class.

(sticky surfaces can be more than 1)


Friction1

friction

  • How does it work?

    It opposes motion.

    It always acts parallel to the surface.

    It cannot cause motion!

    usingFREE BODY DIAGRAMS to show forces

N

F applied

F friction

mg


Free body diagrams vector arrows are used to represent forces

FREE BODY DIAGRAMSVector arrows are used to represent forces.

What force is going to cancel the weight?

NORMAL

FORCE

F applied

F friction

m

Weight (mg)

NORMAL FORCE IS THE FORCE OF A SURFACE ON AN OBJECT.

IT IS ALWAYS DIRECTED PERPENDICULAR TO THE SURFACE.


What if f applied is larger than the frictional force

What if Fapplied is larger than the frictional force?

The resulting force causes

an acceleration.

F normal

F applied

Fnet = Fa - f

Fnet = ma

F friction

m

Weight (mg)


Remember newton s first law inertia and the level of force needed to move a refrigerator

Remember Newton’s first law (inertia) and the level of force needed to move a refrigerator;

Once you get it moving, it is easy to keep moving. Why is that?

PHYSICS!

Ugh!


Sliding friction

Sliding Friction

The constant K is called the “coefficient of kinetic friction”

µK is discovered by pulling with a constant Fapplied

as the object slides at a constant speed.

Fapplied – µkinetic (Fnormal) = ma

if: ma = 0 (no acceleration) then: Fapplied = µkinetic (Fnormal)

or:

µkinetic = Fapplied / Fnormal


Static friction

Static Friction...

The constant µS is called the “coefficient of static friction”

µS is discovered by increasing Fapplied until the

block is at the brink of starting to slide.

Fapplied – µstatic (Fnormal) = ma

if: ma = 0(no motion)then: Fapplied = µstatic (Fnormal)

or:

µstatic = Fapplied / Fnormal


Some coefficients of friction

Some coefficients of friction

static friction kinetic friction

s k

Material on Material

steel / steel0.60.4

add grease to steel0.10.05

metal / ice0.0220.02

brake lining / iron0.40.3

tire / dry pavement0.90.8

tire / wet pavement0.80.7


More on friction friction between road and tires is traction or a dhesion

It takes nearly 25% longer to stop on wet

asphalt as dry

four times longer to

stop on dirt road as dry asphalt.

Wet snow is almost impossible to start,

stop or turn a vehicle on (inertia)

Coefficient of friction

– A. Dry asphalt is .78

– B. Wet asphalt is .60

– C. Dirt road is .20

– D. Dry snow is .20

– E. Wet snow is .10

Types of Traction

There are three types of traction. Each are to

overcome inertia. (rest, motion, straight line)

– A. Acceleration traction

– B. Braking traction

– C. Cornering traction

Things That Can Affect Traction.

A.. Tires - Type, Wear, Inflation

B.. Road Surface - Asphalt., Concrete, Gravel, Dirt

C.. Condition of road – Surface, Rain, Snow, Ice

More Factors that affect Traction:

Weight of vehicle

Type of vehicle

Bank of road/curve

Speed of vehicle

Driver response

Temperature of road/tires

More on friction :Friction between road and tires is traction or adhesion


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