how to know what friction feels like

8/9/2019 How to know what Friction Feels Like

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Feeling Friction:The nature of static and kinetic friction; the coefficient of friction

January 19, 2015

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You will return this handout to the instructor at the end of the lab period.

Table of Contents0. Introduction 1

1. Activity #1: A hands-on investigation of friction 2

2. Activity #2: Numerical values for the forces of static and kinetic friction

. Activity #: !he coefficient of friction". $hen you are done %ith the la&'..10

0. Introduction

Abstract $hy do moving o&(ects sto)*

+ou have &een told that an o&(ect in motion stays in motion if no forces act on it.

,o%ever everyday e)erience may seem to disagree %ith this. $e kno% that %e must al%ays)ress on the gas )edal of an automo&ile to kee) from coming to a sto). +ou kno% %hat ha))ens

to a &icyclist %ho sto)s )edaling. $hy do not these o&(ects kee) moving as Ne%ton )romised*

/ro&a&ly you heard that the force of friction slo%s these o&(ects. o you &elieve this*

riction can &e very difficult to understand &ecause it is hard to o&serve directly. +ou can easilyvisualie eerting a force on a heavy &o to )ush that &o across the floor. 3nfortunately the

&o sto)s moving %hen you sto) )ushing and you say that some force called friction that you

cannot see is res)onsi&le for sto))ing the &o.

In this la& your goal is to measure invisi&le frictional forces as they act on o&(ects &othmotionless and moving.

0.1 ullin! a bloc" with a force sensor

In Activities #2and # you %ill )ull a &lock of %ood &y a string attached to a 4tudent

orce 4ensor. !he 44 %ill monitor the tension in the string. !he follo%ing are im)ortant to

ensure the 44 gives accurate information.

!he string must &e horiontal )arallel to the ta&le to).

!he string must &e )er)endicular to the arm of the force sensor. 4ee igure 1.

a!e 1 # $e %oyne hysics &aculty

Instructions

Before lab, read theIntroductionand

section1.1, and answer the Pre-ab

!uestions on the last "age of this

handout. #and in $our answers as

$ou enter the general "h$sics lab.


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%F%

&ood Block4tring 5 )arallel to ta&le to)

607

Figure 1 SFS, string, and wood block, as seen when looking directly down onto the table top.

1. 'cti(it$ )1: ' hands-on in(estigation of friction

Abstract In this activity you %ill )erform a 8ualitative measurement of &oth static and kinetic friction forces for t%o

different surfaces.

*+ui"ent: A %ood &lock

!%o iron &arsA stri) of adding machine )a)er 9 %ide and a&out "0 cm long

;asking ta)e kno%n as the maimum static friction force > is attained. !hen theo&(ect &egins to move. After the o&(ect has &egun to move there is still a resistance to the

o&(ect?s motion no% due to %hat is kno%n as the force of kinetic friction.

1.2 Feelingstatic friction

1.2.1 /lace the clean %hite adding machine )a)er running from the end of the ta&le

farthest from the com)uter monitor to%ards the other end and ta)e the )a)er securely to

the ta&leto) %ith masking ta)e.

1.2.2 /lace the %ooden &lock on the )a)er %ood side do%n. /lace t%o iron &locks on

to) of the %ooden &lock side &y side. @ently )ush one of the ends of the %ood &lock

)arallel to the ta&leto) %ith one or t%o fingers sensing %hen you have (ust a))liedsufficient force to cause the &lock to &egin to move. !he force you a))lied thatust

barely starts the block mo!ingis the 'a(i'u' static friction


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2. Hn the main menu click on ile and choose H)en. In the %indo% that )o)s o)en select the

file eelingriction.m&l &y highlighting it and clicking H)en. !his file tells the com)uter that a

;otion etector and a 4tudent orce 4ensor have &een connected to the interface &o. +oushould no% see em)ty is)lacement versus !ime and orce versus !ime gra)hs on your screen.

2.G Bali&rations

2.G.1 +ou must cali&rate the motion detector. etailed instructions for doing the

cali&ration are on a sheet of )a)er at each la& ta&le. ,ere is a summary of the series ofmenu selections &eginning %ith the main menu: */"erient, alibrate, Port ,

etails, 2nlock enter the current room tem)erature into the !em)erature &o


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2.6.1 ,ook the 44 to the %ood &lock %ith a short )iece of string as demonstrated &y

the la& instructor. 4ee igure 2 for a sketch of the e)erimental setu).

2.6.2 4tart collecting data &y clicking on the ollecticon on the main menu.

2.6. Another team mem&er %ill )ull the %ood &lock %ith the 44 as descri&ed in the

net section.2.6. !he goal is to have the %ood &lock start moving slo%ly sto) start moving again

sto) etc. !he follo%ing %ill hel) you do it right and get a good gra)h.

,old the 44 so that the hook arm is )er)endicular to the string as sho%n inigure

2.

ee) the 44 slightly a&ove the ta&le so it does not ru& on the ta&le surface as you

slowlydrag the %ood &lock.

/ace yourself &y re)eating O4tart 1 2 P 4to) 1 2 P 4tart 1 2 P 4to) 1 2 P

etc.Q

After )ulling the 44 to%ard you %hile saying 4tart 1 2 you sto) )ulling andlet your hands mo!e slightly away $rom you and toward the wood block so that the

tension in the string falls to ero. If you do this correctly you %ill see the string settleslightly do%n%ard and stay in that )osition until you )ull again.

o not watch the co"uter onitorP %atch the &lock and the string to &e sure the

&lock is moving )ro)erly and the string is fully relaing &et%een )ulls.

2.6.G Aim to get a fe% 4tartL4to) re)etitions &efore the &lock starts to move off the

%hite com)uter )a)er.

Figure -) Your isplacement !ersus /ime and Force !ersus /ime data should look

similar to the abo!e, though with perhaps only two or three peaks instead o$ se!en.

a!e 5 # $e %oyne hysics &aculty


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2.6.M $hen done your data should a))ear similar to those sho%n inigure though

%ith fe%er )eaks. If necessary re)eat 2.6.2 - 2.6.G until you get a good set of data.

2.10 Hnce you have o&tained a satisfactory data set show it to the lab instructor before

"roceeding an$ further.

2.11 After sho%ing your Cogger /ro is)lacement-!ime and orce-!ime )lots to yourinstructor )rint one co)y for each )erson at your ta&le.

1orce )eaks

Aegions of a))ro.imately constant force

Figure 0) Force peaks and regions o$ appro%imately constant $orce

2.12 Cook carefully at your orce versus !ime data. +ou %ill notice )eaks on the gra)h %here

the force takes a maimum value


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" 3 /he steeply rising $orce cur!e that precedes each peak in the Force !ersus /ime graph is

a graph o$ static $riction !ersus time. *stimate about how long it took you to increase the

static $riction $orce $rom 4ero newtons to the ma%imum static $riction $orce. *%panding the

time a%is will help you get a reasonably accurate answer. 5/o e%pand the time a%is, click any

o$ the times on the time a%is 6e%cept $or the $irst and the last, 7 s and -7 s8. /he 9:+%is Scale

dialog bo% will pop up. ;hange the !alues in the


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2.1M.1 Bhoose a!oodconstant force region from your orce-!ime )lot. y!oodone

means a region %here the force is almost a constant and the corres)onding is)lacement-

!ime gra)h is close to a straight line.

2.1M.2 Enclose this constant force region in a &o &y clicking and dragging your mouse

around it. Blick on 'nal$3eand then choose %tatistics.

2.1M. A %indo% %ill a))ear %ith the mean value of the force. 5ecordthis value here.

Average force of kinetic friction for %ood on )a)er: 444444444 newtons

2.1F $hen done do not close the ogger Pro file or shut down ogger Pro. If you do you

%ill have to re-do the cali&ration of the 44.

2.1" ,o%ever dochange the Cogger /ro %indo% so that it sho%s only a single )ane containinga orce versus !ime )lot. !his can &e done through the follo%ing t%o ste)s.

2.1".1 rom the Cogger /ro main menu &ar: 6iew 7ra"h a$out 8ne Pane

89. !his leaves a single )ane dis)laying is)lacement versus !ime.

2.1".2 If the single )ane does not dis)lay the orce versus !ime gra)h click on thevertical ais la&el 5 the la&el reading is"laceent . In the menu that )o)s u) add a

check to Forceand remove the check on is"laceent. !hen click 89.


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$ood &lock )lus t%o

iron &ars )lus 1.2 kg

$ood &lock )lus t%oiron &ars )lus 1.G kg

/able 1

..1 In activity #2 sections 2.1Gand 2.1M you learned ho% to measure


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/able -

..G 3se ;4 Ecel to make a gra)h of&"versus&/. In ;4 Ecel the ta&le of values

for the gra)h %ill look like !a&le %ith&/the left column and&"the right column. !o

the gra)h add a straight line fit


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Pre-ab !uestions

PrintYour Name

______________________________________ead the*ntroductionto this handout and %hen necessary dra% on your o%n e)erience to ans%er the

follo%ing 8uestions &efore you come to @eneral /hysics Ca&. $rite your ans%ers directly on this )age. $hen you

enter the la& tear off this )age and hand it in.

1. If you )ush an unmotoried inanimate o&(ect so that it moves for%ard unless you kee)

)ushing eventually it slo%s to a sto). $hy*

2. $ho first stated that an o&(ect in motion remains in motion %ith the same velocity unless a

force acts on it*

. !hink of )ushing a heavy o&(ect to make it slide across the floor


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10. /lot the follo%ing

how to know what friction feels like

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