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Friction

Pages 23-26 in text

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What is Friction Friction is a force

A frictional force arises when two

substances contact each other.

The molecules of each surface interact

according to Newtons Laws of Motion.

Friction always opposes motion, i.e., it is

opposite to the direction of velocity.

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Types of Friction

Dry FrictionOccurs between the non-lubricated

surfaces of solid objects

Fluid FrictionOccurs with fluids,or

lubricated surfaces

Dynamic FrictionWhen dry friction acts

between two surfaces that are

moving relative to each other

>Static FrictionWhen dry friction acts between

two surfaces that are not

moving relative to each other

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Contact Force

Force that occurs between objects that are in

contact with each other.

Contact forces can be resolved into

components that are perpendicular and

parallel to the surfaces in contact.

The perpendicular component is called the

normal force.

The parallel component is called friction.

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Contact Force in Running

Normal ForceFriction Force

During the push off phase in running, the normal force actsupward on the runner, while the friction force acts forward on

the runner. The frictional force is the only force capable of

moving the runner horizontally down the track. The normal

force can only accelerate the runner upwards.

Resultant force on runner

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Friction and the Normal Force

The maximum frictional force is

proportional to the normal contact force.

An increase in the normal force results in anincrease in the maximum friction.

This is because the molecules on the two

surfaces are pushed together more, thusincreasing their interactions.

d i h d

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Increased Weight, Increased

Normal Force, Increased Friction

5 kg10 kg

Surfaces are more compressed together and there

are more interactions between molecules

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Friction and Surface Area

Friction is not affected by the size of thesurface area in contact.

If the normal force remains constant, but the

contacting surface area is increased, thenthe normal force is spread out over moremolecules, thus the force on each moleculeis reduced.

Amontons (1699)

What about race car tires?

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Calculating Friction

Ff_max = FN

Ff_max is the maximum force of friction

(Mu) is the coefficient of friction

FN is the normal force

Friction can range in value from -Ff_max to +Ff_max

depends on the types of surfaces that are interacting.It would be low for rubber on ice, but high for rubber

on asphalt. It also depends on whether the surfaces are

moving relative to each other ( static ordynamic )

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Friction Example

A 5 kg block of wood rests on a ceramic counter. If the

coefficient of static friction between the block and the counteris 0.4, what horizontal force is necessary to move the block.

5 kgFh

Normal force = FN = mg = 5 x 9.81 = 49 N

FN

mg

Ff

FhFree body

diagram

Fx = maxFhFf= max = 0

Fh = Ff

Fy = mayFNmg = may = 0

FN = mg

Fh= Friction force = FN = 0.4 x 49 = 19.6 N

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Horse Pulling Cart

According to Newtons 3rd Law, these forces are equal and

opposite. So, if the horse pulls forward on the cart with thesame force as the cart pulls back on the horse, how will the

horse ever move the cart?

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SolutionFriction acts on the horses feet but very little acts on the wheels of the

cart. Drawing a free body diagram reveals the answer. The horse and

cart are one system so the forces in between them are internal and

cannot produce a change in motion of the system.

mg

Friction force

resulting from the

horse pulling back

on the ground

Force of friction on

the wheel which

opposes the motion of

the horse-cart system

FN

FN

FF FF

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Tug of War

Fat Bastard vs. Phil PfisterFat Bastard

Pull Force = 3000 N

Mass = 210 kg

Height = 1.8 m

Pfister

Pull Force = 3000 N

Mass = 120 kg

Height = 1.8 m

Both competitors are wearing the same footwear which

has a coefficient of friction of 1.5 with the rubber floorthey are competing on. If both men employ the same

technique, who wins?

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Two Free Body Diagrams

Fat Bastard Pfister

3000 N 3000 N

2060 N

Ff= FN= 1.5 x 2060

= 3090 N

2060 N

1180 N

1180 N

Ff= FN

= 1.5 x 1180

= 1770 N

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Fat Bastard Wins Both competitors have a force of 3000 N

pulling on them from the rope.

Fat Bastards extra mass gives him apotential friction force (3090 N) which isgreater than the force of the rope, so hedoesnt move.

Pfisters maximum friction force (1170 N)is less than the force of rope, so he is pulledtoward Fat Bastard.

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Would it be better to pull up or down

on the rope?

Suppose competitor A was taller than

competitor B.

A would be pulling on an upward angle,

while B would be pulling on a downward

angle.

Who has the advantage?

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Pulling Up On The Rope

mg

FN

Fx1

rope

Fx2

Fy1

This component

increases N

Fy = mayFNFy1mg = may = 0

FN = mg + Fy1

Friction force = Fx1 = FN

Bigger N, means larger

friction force

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Pulling Down On The Rope

mg

FNFx1

rope

Fx2

Fy1

This component

decreases N

Fy = mayFN + Fy1mg = may = 0

FN = mgFy1

Friction force = Fx1 = FN

Smaller N, means less

friction force

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Midterm Example Question

40

y

x

Fx1

A 5 kg box is being pushed up a 40

incline with anacceleration of -2 m/s/s. If the coefficient of dynamic

friction between the incline and box is 0.2, then what

is the value ofFx1? Remember thatfriction alwaysopposes the direction of motion.