physics 121 mechanics

7/27/2019 physics 121 mechanics

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Physics 121, Winter 2013

Lecture Instructor: Nikolai Tolich

Jan 25, 2013


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Reading chapter 51 to 52 FricDon

StaDc FricDon KineDc FricDon

rag Forces and Terminal Speed


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The origin of fricDon iselectromagneDc aracDonforce between molecules/

atoms of one surface tothese of another in closecontact.

The microscopic contacDngsurface area increases whenthe normal force increasesdue to the flaening of theDps.

Polished Steel Surface


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StaDc fricDon is the fricDonal force that preventssurfaces in contact from sliding.

The direcDon of the staDc fricDon is anDparallelto the force trying to slide the object.

While the object is not moving, the magnitude ofthe staDc fricDon equals the magnitude of the

force trying to slide the object unDl it reaches the

maximum value given by

sis the coefficient of staDc fricDon (dimensionless). Fn is the magnitude of the normal force by one

surface on the other.

fs max

= sF

n

FBD of

the block

Fn

WBE

FBM

fs


6/21

A box sits on the horizontal bed of a moving truck.StaDc fricDon between the box and the truck keeps

the box from sliding as the truck accelerates to the

leY as shown. Which of the following diagrams best

describes the staDc fricDonal force acDng on the

box?


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KineDc fricDon is the fricDonal force that opposessliding moDon.

The direcDon of the kineDc fricDon is anDparallel tothe velocity of the sliding object.

The magnitude of the kineDc fricDon is given by

k is the coefficient of kineDc fricDon (dimensionless). Fn is the magnitude of the normal force by one surface on

the other.

fk=

kFn


9/21

A block slides on a table pulled by a stringaached to a hanging weight. In case 1 the

block slides without fricDon and in case 2there is kineDc fricDon between the sliding

block and the table. In which case is the

tension in the string the biggest?


10/21

A 0.11kg hockey puck whose iniDal speedwas 6.0 m/s slides on the ice for 15.0 m

before it stops.a) What was the magnitude of the fricDonal force

on the puck during the sliding?

b) What was the coefficient of fricDon between thepuck and the ice?


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For any given contacDngsurfaces,k is less than or

equal tos.

ou have to push harder toget an object to begin

sliding than to keep it

sliding at constant speed.


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Material s

k

Steel on steel 0.7 0.6

Brass on steel 0.5 0.4

Copper on cast iron 1.1 0.3

Glass on glass 0.9 0.4

Teflon on Teflon 0.04 0.04

Teflon on steel 0.04 0.04

Rubber on concrete (dry) 1.0 0.80

Rubber on concrete (wet) 0.30 0.25

Waxed ski on snow (0oC) 0.10 0.05


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Incline with Sliding Blocks emonstraDon of various surfaces with different

coefficients of staDc fricDon.

NBR

fs BR

WBE

static friction: fs BR

= sN

BR

in x-dir: NBR cos!"WBE = 0

in y-dir: fs BR

cos!" NBR

sin! = 0

s= tan!


14/21

In a rope dangling from a ceiling, tension isthe greatest at the ceiling due to the weight

of the rest of the rope dangling below. If the mass of the rope can be neglected, the

tension is the same throughout the rope.

Applying Newton's 2nd Law,T ! "T = #m

sa

For a rope with negligible mass, #ms= 0.

T = "T


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The force exerted by the fricDonless massless pulleyon the rope is always a normal force.

A normal force has no component tangent to therope, so it cannot produce a change in the

magnitude of tension.


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If the pulley axis is not acceleraDng, thenet force on the pulley axis is zero.

Right Pulley: The weight, W, is pulling the pulley axis down. Two tensions (of the same magnitude, W/2)

are pulling up the pulley axis.

LeY Pulley: Two tensions (of the same magnitude, W/2)

are pulling down the pulley axis.

The support is pulling up the pulley axis withforce W.

If the mass moves up byxcm then wemust pull the rope down by 2xcm.

Mass


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Two blocks are connected as shown on a horizontaltable with the coefficient of kineDc fricDon = 0.10.

The right string is pulled to the right with a force

T2 = 65.0 N. m1 = 12.0 kg and m2 = 24.0 kg.

a) What is the acceleraDon of the system?b) What is the tension, T1?

m1

m2

T1 T

2


19/21

Two blocks are connectedwith a massless rope over a

fricDonless massless pulley.

A block with m1 = 10.0 kgslides down an incline at

= 30.0 with a coefficient

of kineDc fricDon k= 0.20

at a constant speed. What

is the mass of the other

block, m2?


20/21

A fluid exerts a drag force on an object movingthrough it that opposes the moDon.

The faster an object moves in a fluid, the greaterthe magnitude of the drag force acDng on it.

The drag force depends on the shape of themoving object and the viscosity of the fluid.

Without air drag, a falling object keepsacceleraDng unDl it hits the ground.

With air drag, a falling objects reaches a terminalspeed when the gravitaDonal force equals the

drag force, resulDng in no more acceleraDon.

FD


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Terminal velocity in oil

physics 121 mechanics

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