Engineeringskeletons
friction: part 1
practical activity 3 | student instructions | page 1 of 3
A slippery life
Imagine what it would be like without friction in our lives.
If there was no friction between our shoes and the floor we
would find it very difficult to stay upright. With no friction to
hold the stitching on our clothes in place they would fall off
our bodies. Moving cars, buses and planes wouldn’t be able
to stop – and neither would we. Life would be very different
without friction.
However, sometimes it’s essential that friction forces are
kept as low as possible. Think about the moving parts
of machinery or the joints in our bodies. Try rubbing
your hands together for a short time. The friction forces
between your hands soon get transferred into heat. Just
imagine that happening inside your body where two bones
rub together, such as in your hip joint. Nature has done a
very good job of designing our joints to minimise the friction
between the two bone surfaces. But when this natural
good design is upset, for example by injury to the joint or
a disease like arthritis, it’s the job of the bioengineer to
design replacement joints with similar low friction.
WHAT YOU HAVE TO DO
In this investigation you will see how you can measure friction and compare the values
between different types of surface. You will also investigate how you can reduce friction and
discover which surfaces have the lowest friction.
Equipment
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pulley
a length of light string
set of 100 g masses on a mass hanger with a flat base
set of 10 g masses on a mass hanger and some 5 g and 2 g masses
a number of different surfaces: wood, metal, plastic, glass, mouse mat, paper etc.
wax polish
metal polish
WD 40 or light oil
SAFETY NOTES
There are no real safety issues with this investigation. The number of masses used will not be
excessive. Take care when handling glass.
Engineeringeverywhere
Engineeringskeletons | friction: part 1 | student instructions
page 2 of 3
METHOD
1.Clamp the pulley to the edge of the bench
or table.
2.Tie a short length of string (about 30 cm)
to the mass hanger of the 100 g masses.
3.Allow the string to pass over the pulley
and hang down over the side of the bench
or table. Make sure that the string is tied
to the mass hanger at the same height as
the top of the pulley. The string should be
horizontal across the bench to the pulley.
4.Make a loop in the string at the free end.
Attach the other (smaller) mass hanger to this loop. (See photo.)
5.Carefully add masses to the smaller mass hanger until the larger masses on the bench
just start to move towards the pulley. Use the 10 g, 5 g and 2 g masses to find the
combination that will just allow the whole thing to move. Don’t drop the masses onto the
hanger and don’t give the masses on the bench a push.
6.The mass of the load on the bench is called R, the reaction force. The mass hanging from
the string is F – it is the friction force needed to start R moving. Calculate the ratio F ÷ R.
This ratio is called the coefficient of friction, m. You have calculated the value of m between
the bench and the bottom of the 100 g mass hanger.
7.Now add a 100 g mass to the mass hanger on the bench so that now R = 200 g in total.
Repeat steps 5 and 6 to find the coefficient of friction, m.
8.Add another 100 g to R (giving R = 300 g) and repeat steps 5 and 6 again to find m. What do
you notice about the value of m each time?
9.Just to check, repeat the test again using the same apparatus as before.
10.Find a mean (average) value for m.
11.Now repeat the procedure for different surfaces: glass, plastic, metal, felt, mouse mat
etc., using the same set of masses and mass holder as before.
Engineeringeverywhere
Engineeringskeletons | friction: part 1 | student instructions
page 3 of 3
RESULTS
Example results table
Surface used = ______________________
R /g
100
200
300
F /g
m=F÷R
F /g
m=F÷R
Repeat
R /g
100
200
300
Mean value of m (= F ÷ R) ______________________
When the values of coefficient of friction for all the surfaces have been found, put the values in
increasing order of magnitude.
• Which surface has the lowest value of m? Is this the surface that allows the masses to slide easily?
• Look on the Internet for values of coefficient of friction.
• Compare the values given with your own. Are they exactly the same? Are the surfaces in
the same increasing order?
SOME MORE THINGS TO TRY
• Find the surface that will give you the lowest value of coefficient of friction. You could try
polishing the surfaces with furniture or metal polish or using a little oil or WD 40.
• Glue different surfaces to the underside of the mass holder on the bench to find two
surfaces which together give the lowest value of m.
MORE INVESTIGATIONS
• Find out which materials are used to cover or replace the ball and socket in hip
replacement operations.
• During hip and other joint replacement operations, ‘foreign’ materials, for example metals
and polymers, must be placed inside the body. Since these will stay inside the body for
many years, what other properties of these materials need to be considered?
• Investigate hip joints (see friction: part 2).
Engineeringeverywhere
Engineeringskeletons
friction: part 2
practical activity 3 | student instructions | page 1 of 2
replacement hip
If you stand on one leg, you should be able to move your
free leg backwards and forwards, swing it from side to
side and rotate it from the hip. This is possible because
your hip joint is a ‘ball and socket’. The thigh bone is called
the femur and the top of the femur is shaped like a ball.
This ball fits tightly into a cup-like depression in the pelvis
called the acetabulum, allowing full range of movement at
the hip joint.
The acetabulum is lined with cartilage which reduces the
friction as the two bones move against each other. Arthritis
is a disease which wears away the cartilage, causing the
two bones to rub against each other, reducing mobility and
creating stiffness and pain.
WHAT YOU HAVE TO DO
You will make a model of a ball and socket hip joint and note how an increase in friction at the
joint affects movement.
Equipment
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Fimo modelling clay or similar 3 x 25 g
pestle
cling film
varnish and brush
sharp sand
WD 40 or light oil
small knife
baking tray
oven heated to 110 oC
oven gloves
SAFETY NOTES
Do not exceed a temperature of 110 oC to harden the models as harmful gases may be emitted.
Do not use a microwave oven to harden the models. Wash hands after using modelling clay.
Engineeringeverywhere
Engineeringskeletons | friction: part 2 | student instructions
page 2 of 2
METHOD
1.Cover the rounded end of the pestle (the part used to grind) with cling film, to protect it.
2.Warm the Fimo modelling clay in the hands to soften it. Then mould it round the cling
filmed end of the pestle. The aim is to make a shallow cup that just fits around the pestle.
3.Carefully remove the modelling clay from the pestle without squashing it and place it on a
baking tray. Make further cups, as required, in the same way.
4.Dry the models for 30 minutes in an oven at 110 oC to harden them. Remove the baking tray
from the oven.
5.When the model is initially removed from the
oven it will still be quite soft. It will be possible
to trim the top of the cup with a knife to make
it smooth. Take care not to burn your fingers!
6.Leave the model to cool down and harden.
7.The pestle should fit snugly inside the cup
(or socket) you have made. Note how you can
move the pestle (or ball) in all directions inside
the socket. (See photo.)
8.Now stand the pestle upright inside the cup.
Note how far from the vertical you need to
displace the pestle before it falls over.
9.Coat the inside of one of the cups with varnish and sprinkle it with sharp sand. Leave to
dry and then tip out the excess sand from the inside.
10.Now repeat instruction 8. You should find that you can move the pestle further from the
vertical before it falls over. Why is this? Is the pestle easier or harder to move about inside
the socket?
11.Spray another cup with WD 40 or grease with oil. Repeat instruction 8. Do you notice any
difference to the original cup?
12.Follow the link below to find out more about the ball and socket hip joint and how it can be
replaced or resurfaced when damaged.
http://www.hipresurfacing.com/patientarticle.asp?article=30&sect=13
RESULTS
Write a short report about your findings.
Explain some of the features in the design of an artificial hip joint.
Engineeringeverywhere
Engineeringskeletons
friction: parts 1 and 2
practical activity 3 | teacher notes | page 1 of 3
HEALTH AND SAFETY
A risk assessment must be made before starting any practical work. Students will need to
take care with any pieces of glass used.
FRICTION THEORY
The coefficient of static friction between two surfaces, m is the ratio of the friction force, F to
the reaction force, R when the object just starts to move.
The reaction force, R equals the weight of the object, Mg (see diagram). The friction force, F
equals the weight hanging from the pulley, mg when the object just starts to slide.
R (= Mg)
F (= mg)
Mg
mg
So coefficient of friction,
m=
F
R
or
m=
mg
R
If we disregard any friction at the pulley itself, the friction force pulling the object along is the
weight applied to the pulley, mg. Students can calculate the coefficient of friction as
m=
mg
Mg
or
m=
m
M
where M is the mass of the object and m is the mass suspended from the pulley.
The value of m will usually be less than 1.
For small values of M (up to about 300 g) this ratio is constant and doesn’t change with the
value of M. However, if M increases beyond 300 g you may find that the coefficient of friction
starts to decrease. Since friction is quite a complicated property this aspect may be better left
for study at advanced level.
Engineeringeverywhere
Engineeringskeletons | friction: parts 1 and 2 | teacher notes
page 2 of 3
THE INVESTIGATION (Friction: part 1)
Values of the coefficient of static friction, m can be found in data books or on the Internet.
The values will be given for two named surfaces. The experimental values that the students
calculate will depend on the material that the mass holder and the bench are made from and
may not agree exactly with those published. However, the order of the values for different
surfaces should agree with those in published tables. When comparing the values of m for
different surfaces it is important that each group of students working together uses the same
mass holder throughout the investigation.
The practical also works using 50 g brass masses on a mass holder on the bench for M. Use
values of M equal to 100 g, 150 g, 200 g etc. and carry out the procedure as before. Since one
of the surfaces has now changed (from aluminium alloy to brass) the values of coefficient of
friction will have changed too.
If one of the surfaces is wood or a metal, it will make a significant difference if the surface is
polished or not. The students could be left to discover this themselves and this fact can be
used to determine the surface with the lowest value for m.
Also it is important to note that this investigation is into the coefficient of static friction. In
other words the object is not moving at the start of the investigation. There is another property
called the coefficient of kinetic friction where the object is already moving when the pulling
force is applied. This may be tested by carrying out the instructions as in the student notes
but giving the mass holder on the bench a small push each time a new mass is added to the
pulley. The coefficient of kinetic friction should be found to be less than the coefficient of static
friction for the same surfaces.
Sample results
These results are for a 100 g aluminium alloy mass holder on the bench but with different
surfaces between the bench and the mass holder.
Wood (polished)
Wood (unpolished)
R /g
100
200
300
F /g
30
60
90
m=F÷R
0.30
0.30
0.30
Paper
R /g
100
200
300
R /g
100
200
300
F /g
15
25
40
m=F÷R
0.15
0.15
0.13
Cling film
F /g
50
90
150
m=F÷R
0.50
0.45
0.50
R /g
100
200
300
F /g
60
140
200
m=F÷R
0.60
0.70
0.67
These results are all using a 50 g brass mass holder on the bench but with different surfaces
between the bench and the mass holder.
Wood (unpolished)
R /g
100
200
300
F /g
25
40
45
Engineeringeverywhere
Wood (polished)
m=F÷R
0.25
0.27
0.23
R /g
100
200
300
F /g
15
25
30
m=F÷R
0.15
0.17
0.15
Engineeringskeletons | friction: parts 1 and 2 | teacher notes
page 3 of 3
Tables of values for coefficient of static friction can be found at:
http://www.physlink.com/Reference/Frictioncoefficients.cfm
http://www.engineersedge.com/coeffients_of_friction.htm
http://frictioncenter.siu.edu/databaseSearch.html
http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm
Also details of materials used for hip replacements including the Birmingham hip at:
http://www.hipresurfacing.com/surgeonarticle.asp?article=6&sect=6
http://www.zimmer.co.uk/z/ctl/op/global/action/1/id/1461/template/PC/navid/556
http://www.doitpoms.ac.uk/tlplib/bones/head.php
For details of commercial friction testing apparatus:
http://www.lloyd-instruments.co.uk/testtypes/coefficient.cfm
THE INVESTIGATION (friction: part 2)
This investigation gives students the opportunity to think about friction in practice. By
extending its brief, a variety of assessment requirements for communication and science in
society may be met.
Time required
Each part of this activity will take one session to complete. The material used in part 2 will
need 30 minutes drying time, so this part might be started before commencing part 1.
TECHNICIAN EQUIPMENT LIST
per group
FRICTION: part 1
•
•
•
•
•
pulley
a length of light string
set of 100 g masses on a mass hanger with a flat base
set of 10 g masses on a mass hanger and some 5 g and 2 g masses, if possible
a number of different surfaces: wood, metal, plastic, glass, mouse mat, paper etc. (metal
should not have any sharp edges; glass will need to have sharp edges taped)
• wax polish
• metal polish
• WD 40 or light oil
FRICTION: part 2
•
•
•
•
•
•
•
•
•
•
Fimo modelling clay or similar 3 x 25 g
pestle
cling film
varnish and brush
sharp sand
WD 40 or light oil
small knife
baking tray
oven heated to 110 oC
oven gloves
Engineeringeverywhere