unit 6 friction

7/28/2019 Unit 6 Friction

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-A force that opposes

motion-Acts parallel to the

surfaces in contact.


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Friction Force vs. Weight

W

Ff

Cause of Friction:themicroscopic roughness between

surfaceslike two gears lockingtogether.

Cause of Friction


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walkingtires

brakes

sand on icy streets


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If it is not moving, then there is no

friction until an applied force acts on

the object! What if it is moving?

Friction can also cause heat!

FFf
http://images.google.com/imgres?imgurl=http://www.abc.net.au/juniors/pages/2000/transport/land/img/friction.jpg&imgrefurl=http://www.abc.net.au/juniors/pages/2000/transport/land/activity.htm&h=250&w=150&sz=31&tbnid=JnlnMye0bd4J:&tbnh=106&tbnw=63&hl=en&start=4&prev=/images%3Fq%3Dfriction%26svnum%3D10%26hl%3Den%26lr%3D


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1. Sliding Frictionoccurs when two

solid objects slide over each other.


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1. Static Frictionnot moving

2. Kinetic Friction - moving

Static > Kinetic

So it takes more force to start

moving an object


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Force vs. Time

t

F

Maximum Static Friction

Constant Kinetic Friction


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Factors that affect Sliding Friction:

a. The normal Force acting on an object. FN

(Fg only equals FN in a special circumstance, ie. lab)

b.The coefficient of Friction ( k< s,unitless) The texture of the surfaces. The

stickiness between surfaces.


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General Formula for Friction:

friction force = coefficient of friction x normal force

or

Ff= F

NStatic - sKinetic -

k


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Which object would have the

greatest normal force?

So which would experience the

greatest friction force? (assume allblocks are moving)


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Which would have greater frictionforce sand paper or a waxed floor?

Why?


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What about surface area? Which would

have a greater friction force?

Friction force does not depend on thearea of contact. Same FN and !


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2. Rolling Friction(< sliding)

wheels and ball bearings


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3. Fluid Friction(< sliding)

occurs when a solid object moves

through a gas or a liquid substance

Liquids


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Gases

Hammer Feather


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Now for the problems

just like beforedraw your FBD

Then:

Fx = ma Fy = ma Ff= FN

but now we need:


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Blockhawk -

Stephen Hawking is moving a 100 kgblock from one side of his laboratory

to the other. To accomplish this he ties

a rope to the block and then ties it tohis wheelchair such that the rope is

pulled horizontally to the floor. If the

coefficient of sliding friction is 0.35,

what force must Mr. Hawking apply

to the rope to move at constant speed?

Ans: 343 N


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Warm Up

1. A 30 N block has a coefficient of frictionof 0.26. What force is required to slide it

along a level surface at constant velocity?

2. A 35 kg crate requires a force of 110 N

to slide at an acceleration of 2 m/s2. What

is the coefficient of friction?


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In the figure below the coefficient of friction is

0.25. Find the tension in the rope then find the

acceleration of the 18 N block.


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A crate having a mass of 60 kg falls horizontally off

the back of a truck which is traveling at 30 m/s.

Compute the coefficient of kinetic friction betweenthe road and the crate if the crate slides 45 m on the

ground with no tumbling along the road before

coming to rest. Assume that the initial speed of the

crate along the road is 30m/s.

30 m/s

Ans. 1.02


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Atwood Again

A cord running over a pulley connects

two objects. The coefficient of friction

between the object and the table is0.33. Find the acceleration of the

system and

the tension ofthe rope.

7 kg

5 kg


24/37

Angle Block Problem:

A 40 kg lawnmower is being pushed at a

constant speed along the ground. The

angle at which the force is being appliedis 35 below the horizontal. What force

will keep the lawnmower moving at a

constant velocity if the coefficient ofkinetic friction is .3?


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How does pulling a block at an upward

angle change the normal force?

friction force?

Fg

FNF

y

x


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How does pulling a block at a

downward angle change the normal

force? friction force?

Fg

FN

F

y

x


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If all the boxes are the same, which

requires a greater force to pull? Why?

F

F F


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Warm-up

A 925 N crate is being pulled across alevel floor by a force of 325 N at an angle

25 above the horizon. The coefficient of

kinetic friction between the crate and thefloor is 0.25. Find the acceleration of the

crate.

925 N

25

325 N

Answer: 1.034 m/s2


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What if were on an incline?

If you were sledding down a hill would mass affect your acceleration?


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There are always at least

two forces acting upon any

object that is positioned on

an inclined plane - theforce of gravity and the

normal force. The force ofgravity (also known as

weight) acts in a downward

direction; yet the normalforce acts in a direction

perpendicular to the

surface.


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FN

Fg

F

FfFg-y

Fg-x

30

- Break Fg into x and y components

- Angle between Fg and y is equal to the angle of the incline

- Rotate the x & y axis

I li d t b F i ti


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Inclined to be Friction

Old Grandma Jones is trying to load a

wooden crate full of apples into hertruck. To accomplish this, she plans to

slide the crate up along a board. If the

crate has a mass of 50 kg and the boardis at a 30 incline, what is the maximum

force she must apply parallel to the

board to move the crate at constantvelocity into her truck if the coefficient of

friction is .25?

Th t DEAD C t!


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That Darn Cat!David is speeding down a hill (incline) when he

notices a cat in the road. He immediately

switches from the gas pedal to the brakes in

order to stop the car. The slope of the incline is

20 and the car has a mass of 1500 kg.

a. Draw a force diagram showing all the forces acting

on the car.

b. Determine the acceleration of the car if the

coefficient of friction between the road and thetires is 0.55.

c. If the cat is 25 m away, and David is going 10 m/s

when he slams on the brakes, does he hit the cat?

That DEAD Cat!


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Quiz part 21. The block given an initial velocity of 10 m/s is sliding

down the hill and will come to a stop due to friction

(k=.7). Hint (The only force acting on the block down the hill is Fgx)

a. Draw a free body diagram of the block.

d. How far will it travel

before it stops?

b. Write the Newtons 2nd law equations.

c. Find the acceleration of the block


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Atwood Again

A cord running over a pulley connects

two objects. The coefficient of friction

between the object and the table is0.33. Find the acceleration of the

system and

the tension ofthe rope.

7 kg

5 kg

Quiz: Part 3


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The brakes are slammed on a speeding

truck and it skids to a stop. If the truckwere heavily loaded so it had twice the total

mass the skidding distance would be:

a) the sameb) 1 times as far

c) twice as far

d) four times as far


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Answer: a) the same

Twice the mass means the skidding

tires will bear against the road with

twice the force, which results in twicethe friction. Twice as much friction

acting on twice as much mass

produces the same deceleration andhence the same stopping distance.

unit 6 friction

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