Download Operational Manual

INSTR Ucn ON MANUAL
HFN.l
Friction on an
Inclined Steel Plane
Friction on an Inclined SteelPlane
INTRODUCTION
Friction - help or hindrance?Cars and trainsneedbrakesto enablethem to stop, but friction in
their mechanisms
wastesenergy. A motor bicyclecan run round a comer becauseof the friction
betweenthe tyres andthe road surface,but its top speedis governedby the wind resistanceof the
driver andvehicle. Friction is the outcomeof one surfaceslidingover another(sliding friction), or
it may also be the maximumresistanceprior to the start of sliding (static friction). The friction
force acts in the planeof the surfacesand experimentshowsthat it is proportionalto the force
holding the surfacestogether. The ratio of the friction force to the normal force is called the
coefficientof friction for the pair of surfaces.
Friction is affected greatly by any lubricant betweenthe surfaces,so for the purposeof this
experimentthe surfacesmust be clean,dry and free from grease. Neverthelessit is possiblethat
the surfaCemay containa dry lubricantin the material;for examplecast iron containsgraphitein
the form of minute particles of carbon. Plastic compoundsare also available like PTFE
impregnatedwith molybdenumdisulphide.
At the other end of the scalecomematerialsfor brakesandmotor car clutches. Herethe object is
to developa high coefficientof friction, andto makesurethe heatgenerateddoesnot destroythe
material. The roughnessof the matingsurfacesmust alsobe a factor, and as a consequence
wear
becomesan issue. The subjectof friction with all its variablesandconsequences
comesunderthe
generalheadingof tribology. Howeverthis experimentdealssimplywith the measurement
of the
coefficientof friction betweenpairsof surfacesat zero or low speeds.
HFN./. Page /.
Issue/. July. 1993.
LIST OF PARTS
The
standard
set
of
items
supplied
~.1)
c.,
consists
- Adjustablesteelplanecompletewith base
- Cord and0.5 N load hanger
- Tray with Nylon liner
of:
I - Tray with brassliner
I - Tray with aluminiumliner
I - Tray with 'ferodo' liner
The weight set supplied(HFN.lw) consistsof 1 x ION, 4 x 5 N, 4 x 2 N, 1 x 1 N,
0.2 N and 1 x 0.1 N.
x 0.5 N, 2 x
Optional Extras
HFN.la
HFN.lc
HFN.ld
HFN.le
Tray with 'Tufnor liner
Tray with ball bearingwheels
Woodensliderblock
P.T.F.E.liner tray
APPARATUS
A cold formed channelsectionof stainlesssteel is pivoted at the centre on an axle carried on a
supportingbase. A protractorattachedto the channelshowsits angleof inclinationand enablesit
to be clampedat that angle. The baseplate has three levelling feet so that the channelcan be
madehorizontal. At one end of the channelis attacheda pulley to turn a towing cord from the
longitudinallay of the channelto a verticalload hanger.
A rangeof trays is availableto provideflat undersurfaces
of differentmaterials. There is also one
tray fitted with threebail bearingwheels. In additionthereis a woodenblock designedto be used
on threesideswhich presentdifferentareasof contactto the steelchannel.
EXPERIMENT
Thereare two experiments
that can \>ecarriedout on this apparatus,both of which dealwith first
principles. Onedealswith the natureof friction on a level surface,while the other extendsthis to
examinefriction on an inclinedplane.
FRICnON BETWEEN TWO SURFACES
The objectsof this experimentare to showthat friction is proportionalto the normalforce, and to
determinethe coefficientof friction betweenvariousmaterialsanda steelplane.
PROCEDURE
As explainedabove,the surfacesusedin this work must be cleanedfor the experimentand kept
free from corrosionwhen not in use. The adjustablesteel plane is to be positioned on a firm
benchso that the load on the hangerpassesthe edgeof the benchas it descends.Clampthe plane
in the 00 position and use a spirit level to set the plane truly horizontal by adjustingthe three
levellingfeet. All the traysto be usedmustbe weighedandtheir massesrecorded.
HFN.I_Lab. Page 2.
1.\:\'UeI. July. 1993.
Part 1
Placethe aluminiumtray on the horizontalsteelchannelat the end remotefrom the pulley. Attach
the towing cord and arrangeit over the pulley with the load hangersuspended.Add load to the
hangeruntil the tray will continueto slide at roughly constantvelocity after beinggiven a slight
pushto start it moving. Recordthis load in table 1.
Table
Mass of tray =
kg.
Repeatthe aboveprocedurewith four incrementsof 5 N placedin the tray
Part 2
With as many trays as are availableusedin turn gently add weights to the load hangeruntil the
stationarytray plus a 5 N load suddenlymoves. Recordthe load as the static friction hangerload
in table2. Then,while eachtray is in use,repeatthe initial procedureof Part 1 (tray still with 5 N
load) andrecordthe resultsin table2
Table 2
Friction coefficients on a steel surface
If the tray on ball bearingwheelsis availabletry to measurethe equivalentof the sliding friction.
It may be necessaryto load the tray as much as possible(for examplewith other trays) to get a
result. To reducethe towing force removethe load hangerand slip the slotted smallweights into
the loop of the cord.
The block of wood is an exampleof a non-isotropic material as the natural growth creates
structure in three axes (radial, tangentialand longitudinal). Dependingon how the block is
orientedto the grain of the wood so the surfacesmayvary. Henceit is worth measuringthe static
and sliding friction on the three sides. Further to this, measurethe areasof contact and then
repeatthe sliding friction experimentwith a 15 N load on the block to seeif the areaof contact
hasanyeffect.
RESUL1'8
Convertthe massof eachtray into its weight multiplyingkg x 9.81 to give Newtons. The normal
force is thenthe weight of the tray plus anyaddedload. The slidingforce is the sumof the hanger
and its addedload. Plot the resultsof Part 1 on a graph of sliding force againstnormal force.
Completetable2 in a similarway.
HFN. I _Lab. Page 3.
Issue I. July. 1993.
OBSERVATIONS
Wasthe coefficientof friction independentof the normalforce in Part I?
Commenton the differencebetweenstaticandsliding friction
FRICTION ON AN INCLINED PLANE
OBJECT
The object of this experimentis first to find the angle of friction of various materialson a steel
plane. The secondobjectis to verify that the force requiredparallelto an inclinedplaneto movea
body up the planecorrespondsto the friction coefficient(or angle)alreadyfound.
PROCEDURE
As explainedabove,the surfacesusedin this work must be cleanedfor the experimentand kept
free from corrosion when not in use. The adjustablesteel planeis to be positionedon a firm
benchso that the load on the hangerpassesthe edgeof the benchas it descends.Clampthe plane
in the 0° position and use a spirit level to set the plane truly horizontal by adjustingthe three
levellingfeet. All the traysto be usedmustbe weighedandtheir massesrecorded.
Part 1
Placethe aluminiumtray on the middleof the steelplanewithout the towing cord attached. Hold
the pulley end of the planeand slackenthe clamp so that the end can be slowly raisedto tilt the
plane. As soonasthe tray startsto slidenote and record the angleof inclinationin table 3. Then
reducethe tilt and replacethe tray at the middle. Now as the tilt is increasedkeepgiving the tray
a slightpushuntil it keepsmoving. Recordthis asthe angleof inclinationfor sliding friction.
Repeatthe procedurefor asmanytrays asare available.
Table 3
Friction an~es on a steel plane
Part 2
Clampthe planeat 10°inclination. Placethe Nylon tray at the lower end and put the towing cord
and load hangerin position to pull the tray up the plane. Add load to the hangeruntil the tray,
given a slight push,slidesslowly up the plane. Repeatthe procedurewith a ION weight in the
tray. Recordthe resultsin table4.
Repeatthe aboveat anglesof inclination20°,30° and 40°.
HFN.I_Lab. Page4.
Issue 1. July. 1993.
Table 4
Friction forces on an inclined plane
RESULTS
Table3 is basedon the equationof equilibriumof a body on an inclinedplane. At the momentof
sliding,or at uniform velocity, the friction force must be equalto the componentof the weight
actingdowntheplane.If thecoefficientof frictionis J.1then
~.
which
wcose= Wsin8
leadsto the conceptof the angleof friction e since
J.l= tane
Completetable 3, and if there are any valuesfor the coefficientsof friction from the previous
experimenton a horizontalplanecomparethe results.
The theoryfrom which table4 is developedis an extensionof the above In this casethe net force
actingup the planemustbe equalandoppositeto the friction force.
P
- Wsine = ~wcose
This can be rearranged either in tenns of P or 1.1. As the experimentis essentiallyabout the
coefficient of friction that determinesthe choice.
To completetable 4 convert the massof the tray to its weight and add any extra load. Finally
averagethe coefficientsof friction for comparisonwith previousvalues.
HFN.l_Lab. Page 5.
Issue I. July, 1993.
OBSERVATIONS
Do the experimentalresultsverify the theory? If thereare discrepanciesin the measuredvaluesof
friction coefficients.which experimentalmethodappearsto be more reliable?
HFN.J_Lab. Page6.
Issue I. July. /993.