Road Safety
Science
Every year some hundreds of people are killed on the roads
in Ireland – about one a day on average. Awareness campaigns
have not produced the desired change in driver behaviour.
An understanding of the factors that are common to many
road accidents and an understanding of the laws of motion
Every
some
hundreds
people
killed on the roads
may
help year
increase
awareness
of the of
dangers
andare
encourage
young
people– about
to take one
greater
careon
onaverage.
the road Awareness
whether as campaigns
in Ireland
a day
pedestrians, cyclists, passengers or drivers.
Drivers are advised to leave at least a ‘t
In three seconds a car travelling at 80
speed it would require a very fast re
coefficient of friction (0.9) to bring a ca
50 m. These estimates do not take acc
which is about 200–300 ms.
tyres and the road; when this happens the car cannot be steered and
Reaction time (normal reaction time): the
the stopping distance increases.
of factors
auditory
stimuli
it time
is are:
Other
that affect
reaction
Road
The estimated stopping distance for
time of say 2 s and a coefficient of fricti
have not produced the desired change in driver behaviour.
An understanding
of thereaction
factors that
are common to many
Reaction
time (normal
time):
road
accidents
and an understanding of the laws of motion
The
average
time for
reaction
to visual
stimuli awareness of the dangers and encourage
may help
increase
(with no choices) is
young180people
between
and 200 to
ms take greater care on the road whether as
(0.18–0.20 s). In the case
pedestrians,
cyclists,
passengers or drivers.
of auditory stimuli it is
Fig. 3 Adverse weather can create dangerous driving conditions.
shorter (140–160 ms). When
decisions are required the
time required to make an
appropriate
response to
a
The average
time
for
visual stimulus increases
reaction to visual stimuli
with age from about 370
(with
noyear choices)
is1 Scene of a crash – a common
Fig.
ms,
for 20–30
olds, to
on Irish roads
between
180
200 occurence
ms
about
440 ms
for and
60–70
year
olds.
(0.18–0.20
s). In the case
and
and
Science
Kinetic energy (Ek ) = ½m v 2
Work done = frictional force × distance = F × s
Frictional force is equal to the coefficient of friction multiplied by the
weight of the car: F = µ × mg
Tyre inflation
The frictional force between two surf
involved and on the force between th
does not depend on the contact are
case of a car, the force between the s
distributed over the four wheels. If the
weight of the car presses on a small a
the same but it is transferred through a
frictional force is then more likely to ex
the rubber together and bits of the ty
wider tyres are desirable.
Over-inflation of tyres reduces the area
tyre is more likely to be abraded by
steering more difficult.
Higher speed reduces traction (grip o
probability of aquaplaning in wet condi
In the case of cars that are in good working order travelling on a dry road
Fig. 3 Adverse weather can create dangerous driving conditions.
the coefficient of friction (µ) ranges from about 0.6 (for ordinary cars) to
Alcohol
shorter (140–160 ms). When
about 0.95 (for the most expensive cars).
• the complexity of the situation.
decisions are required the
= F tyres
× s and the road; when this happens the car cannot be steered and
In braking
Ek the
• the illumination (dim situations require more time to more interpret).
time required to make an
½ m v 2 the
= (µstopping
× mg) × sdistance increases.
• appropriate
a driver’s general
level of alertness.
response
to a
Therefore
s = v 2Kinetic
/(2 × µ ×energy
g),
(Ek ) = ½m v 2
stimulus
increases
• visual
normal but
unexpected
events (e.g. brake lights on the car ahead).
where g is the acceleration due to gravity.
with
age
from
about
370
Work done = frictional force × distance = F × s
• unusual and unexpected events (e.g. car approaching on the wrong
Fig. 1 Scene of a crash – a common
ms,
20–30
This shows that the braking distance (s) is proportional to the speed
sidefor
of the
road).year olds, to
Frictional
force is equal to the coefficient of friction multiplied by the
2
occurence on Irish roads
squared (v ); e.g. doubling the speed increases the stopping distance by
about 440 ms for 60–70
weight
of car
theiscar:
= µ ×Formg
• blood alcohol level, which is a factor in about 33% of road accidents,
a factor of four. The mass
of the
not aFfactor.
an ordinary car
year
olds.reaction time and impairs judgement.
increases
travelling at 100 km/hour (27.8 m/s) the braking distance would be about
In
the
case
of
cars
that
are
in
good working orderFig.
travelling
on
a dry
4 Every year
more
then road
300
55 m; this does not take account of the driver’s response time.
Many
studies
have that
shown
that 95%
of real response
times of drivers are
Other
factors
affect
reaction
time are:
people are killed on our roads.
the
coeffi
cient
of
friction
(µ)
ranges
from
about
0.6
(for
ordinary
cars)
to
in the range 1.5 to 2.5 seconds; the faster responses were recorded
In wet conditions the coefficient of friction is about 40% less and if
about 0.95 (for the most expensive cars).
when
drivers
were warned
thatsituation.
there might be a hazard and were
• the
complexity
of the
the wheels lock it may be as low as 0.1, increasing the braking
therefore in a state of readiness.
distance dramatically.
•
the illumination (dim situations require more time to more interpret).
•
Speed
and
a driver’s general level
of alertness.
distance
braking
In braking
Two examples
Ek = F × s
½ m v 2 = (µ × mg) × s
In
o
s
a
p
ti
c
s
c
(0
0
s
2
m
the body mass the higher the BAC. In
drivers varies from zero (Czech Repub
UK) but is mostly 50 mg/100 ml or 0.05
In 2003, 36.5% of fatal road accidents in
Consider the followingTherefore
two situations:
s = v 2/(2 × µ × g),
report at http://www.rsa.ie/publication/
normal but unexpected
brake
on the car 1.ahead).
Theevents
stopping(e.g.
distance
for a lights
car depends
Joe is an average driver, alert and driving at 80 km/h (22.2 m/s) on a
where g is the acceleration due to gravity.
on driver reaction time, the speed of
Driving requires constant attention and
main road in daylight.
• unusual and unexpected
events
(e.g.
car
approaching
the car,
the slope
and
condition
of the on the wrong
The ability to respond appropriately to
This
shows
that
the
braking
distance
(s)
is
proportional
to consumption
the speed of alco
2.
Pat
is
an
average
driver,
tired
and
driving
at
80
km/h
on
a
poorly
lit
side of the road).
road surface, the condition of the tyres
compromised by
2
minor
road
late
at
night.
the stopping
distance
squared (v ); e.g. doubling the speed increasesmobile
and the brake system of the car. In
phones. Every
year by
300–400 p
• blood alcohol level, which
is aa car
factor
in its
about
33%
of road accidents,
bringing
to rest
kinetic
energy
about 1200
are seriously
factor(e.g.
of afour.
The
mass
of the
car iswillnot a factor.
For an
ordinaryinjured
car and o
If an unexpected eventaoccurs
person
walks
out onto
the road)
must
be dissipated
in some way; ideally
is important to ensure your tyres an
increases reaction time
and
impairs judgement.
they be able to avoid atravelling
collision? at 100 km/hour (27.8 m/s) the brakingItdistance
would be about
it generates heat in the brakes. If the
good condition in order to protect you a
55
m;
this
does
not
take
account
of
the
driver’s
response
time.
Joe’s
response
time
might
be
expected
to
be
about
1.5
s;
during
that
are applied
hard that times
the
Many studies have shownbrakes
that 95%
of realsoresponse
of drivers are
•
Fig. 2 Skid marks left by
in the range
emergency
braking.1.5
the car travels 33 m. Once the brakes are applied the car travels a
wheels cannot
then the
car skids were time
to 2.5 seconds;
theturn
faster
responses
recorded
Inbrought
wet conditions
thetotal
coeffi
cientdistance
of friction is about 40% less and if
further 37 m before it is
to a stop. So the
stopping
and frictional energy is dissipated in
when drivers were warned that there might be a hazard and
wereµ=0.7) is 70
(assuming
m. wheels lock it may be as low as 0.1, increasing the braking
the
therefore in a state of readiness.
distance dramatically.
Fig. 2 Skid marks left by
emergency braking.
Speed and braking
distance
Two examples
The stopping distance for a car depends
on driver reaction time, the speed of
the car, the slope and condition of the
road surface, the condition of the tyres
and the brake system of the car. In
bringing a car to rest its kinetic energy
must be dissipated in some way; ideally
it generates heat in the brakes. If the
brakes are applied so hard that the
wheels cannot turn then the car skids
and frictional energy is dissipated in
1. Joe is an average driver, alert and driving at 80 km/h (22.2 m/s) on a
main road in daylight.
Consider the following two situations:
2. Pat is an average driver, tired and driving at 80 km/h on a poorly lit
minor road late at night.
If an unexpected event occurs (e.g. a person walks out onto the road) will
they be able to avoid a collision?
Joe’s response time might be expected to be about 1.5 s; during that
time the car travels 33 m. Once the brakes are applied the car travels a
further 37 m before it is brought to a stop. So the total stopping distance
(assuming µ=0.7) is 70 m.
and
Science
Road Safety
IN
AND
SCIENCE TECHNOLOGY
SECOND
EDITION
EDITION
ACTION THIRD
d Safety
Every year some hundreds of people are killed on the roads
in Ireland – about one a day on average. Awareness campaigns
have not produced the desired change in driver behaviour.
An understanding of the factors that are common to many
road accidents and an understanding of the laws of motion
The
estimated
stopping
distance
for Pat’sand
car,encourage
assuming a response
may help
increase
awareness
of the dangers
time
of people
say 2 s to
and
a coeffi
cient
of friction
of 0.4,
would be
young
take
greater
care
on the road
whether
as 107 m.
pedestrians,
cyclists,
passengers
or
drivers.
Drivers are advised to leave at least a ‘three second’ gap between cars.
In three seconds a car travelling at 80 km/h covers 66 m and at that
Reaction
time
(normal
reaction
time):
speed
it would
require
a very
fast response
time (1 s) and a high
The average
time for(0.9) to bring a car to an unanticipated stop within
coeffi
cient of friction
reaction to visual stimuli
50
m. These estimates do not take account of brake engagement time
(with no choices) is
which
about
betweenis180
and 200–300
200 ms ms.
The estimated stopping distance for
time of say 2 s and a coefficient of fricti
Drivers are advised to leave at least a ‘t
In three seconds a car travelling at 80
speed it would require a very fast re
coefficient of friction (0.9) to bring a ca
50 m. These estimates do not take acc
which is about 200–300 ms.
Tyre inflation
The frictional force between two surf
involved and on the force between th
does not depend on the contact are
case of a car, the force between the s
distributed over the four wheels. If the
weight of the car presses on a small a
the same but it is transferred through a
frictional force is then more likely to ex
the rubber together and bits of the ty
wider tyres are desirable.
(0.18–0.20 s). In the case
of auditory stimuli it is
Fig. 3 Adverse weather can create dangerous driving conditions.
shorter (140–160 ms). When
decisions are required the
the tyres and the road; when this happens the car cannot be steered and
The
frictional
between two surfaces depends on the materials
time required
to force
make an
the stopping distance increases.
involved
on the
appropriateand
response
to aforce between them. Contrary to expectation, it
Over-inflation of tyres reduces the area
2
visual not
stimulus
increases
does
depend
on the contact area – at least not directly. InKinetic
the energy (Ek ) = ½m v
tyre is more likely to be abraded by
with age
370 between the surfaces is the weight of theWork
steering more difficult.
case
of afrom
car,about
the force
car,done = frictional force × distance = F × s
Fig. 1 Scene of a crash – a common
ms, for 20–30 year olds, to
Frictional
force is equal to the coefficient of friction multiplied by the
distributed
over
the
four
wheels.
If
the
wheels
are
very
narrow
then
the
occurence
on
Irish
roads
Since
January
2000
all
cars
four
years
and
older
in Ireland
must traction (grip o
Higher
speed reduces
about 440 ms for 60–70
weight of the car: F = µ × mg
probability
aquaplaning
weight
year olds.of the car presses on a small area. The frictional force remains
undergo a test called the National Car Test [NCT].
Thisoftest
was in wet condi
In
the
case of cars that are
in good working
order travelling
on a dry road
the
same
but
it
is
transferred
through
a
small
amount
of
rubber.
A
large
introduced
to
improve
road
safety,
to
reduce
harmful
emissions
Other factors that affect reaction time are:
coefficient of friction (µ) ranges from about 0.6 (for ordinary cars) to
frictional force is then more likely to exceed the internal forces the
holding
and to comply with the EU Directive 96/96/ECAlcohol
which makes car
about 0.95 (for the most expensive cars).
• the complexity of the situation.
the
rubber together and bits of the tyre are worn off. For this reason
In
testing compulsory in all EU member states.
In braking
Ek = F × s
o
• the illumination
(dim situations require more time to more interpret).
wider
tyres are desirable.
s
½ m v 2 =Following
(µ × mg) × a
s competitive tender process, the SGS Group was
• a driver’s general level of alertness.
a
Over-inflation of tyres reduces the area of contact with the road Therefore
and the
s =awarded
v 2/(2 × µ ×the
g), contract to operate and manage the car testing
p
• normal
but unexpected
(e.g. brake by
lights
on the carUnder-infl
ahead). ation makes
tyre
is more
likely toevents
be abraded
friction.
service
in
Ireland.
NCTS
now
operates
43
Test
Centres
ti
where g is the acceleration due to gravity.
steering
difficult. events (e.g. car approaching on the wrong
• unusualmore
and unexpected
throughout the country. The SGS Group is the largest inspection,
c
This shows that the braking distance (s) is proportional to the speed
side of the road).
s
testing and verification company in the world.
squared
Higher speed reduces traction (grip on the road) and increases
the(v2); e.g. doubling the speed increases the stopping distance by
c
• blood alcohol level, which is a factor in about 33% of road accidents,
a
factor
of
four.
The
mass
of
the
car
is
not
a
factor.
For
an
ordinary
car
probability
of aquaplaning in wet conditions.
(0
Although this lesson has been about braking, the NCTS tests
increases reaction time and impairs judgement.
travelling at 100 km/hour (27.8 m/s) the braking distance would be about
0
Fig. 4 tyres,
Every yearlights,
more then 300
all
physical
aspects
of time.
car safety: wheels,
55
m;
this
does
not
take
account
of
the
driver’s
response
Many studies have shown that 95% of real response times of drivers are
s
people are killed on our roads.
steering,
electrical
systems,
mirrors,
fuel
systems
etc.
The
in the range 1.5 to 2.5 seconds; the faster responses were recorded
2
In wet conditions the coefficient of friction is about 40% less and if
standards
for
exhaust
emissions
for
both
petrol
and
diesel
when drivers were warned that there might be a hazard and were
m
wheels lock it may be as low as 0.1, increasing the braking
International studies of thethe
effects
therefore in a state of readiness.
the body mass
the higher
the BAC. In
effects
of
distance dramatically. engines are designed to minimise the harmful
of alcohol on driving show
drivers varies from zero (Czech Repub
atmospheric pollution.
UK) but is mostly 50 mg/100 ml or 0.05
significant
impairmentTwo of
examples
Tyre inflation
The National Car Testing Service
Alcohol
Speed and braking
distanceattentiveness,
The NCTS employs about 450 people in vehicle
inspection,
visual function,
In 2003, 36.5%
of fatal road accidents in
Consider the following two situations:
customer service, technical and administrative
roles.
report
at http://www.rsa.ie/publication/
perception and choice reaction
The stopping distance for a car depends
1. Joe is an average driver, alert and driving at 80 km/h (22.2 m/s) on a
time time,
(the
time of requiredmain
toroad in daylight.
on driver reaction
the speed
Driving requires constant attention and
choose
a response
the car, the slope
and condition
of the to a complex
The ability to respond appropriately to
2.
Pat
is
an
average
driver,
tired
and
driving
at
80
km/h
on
a
poorly
lit
road surface, situation)
the condition of
the tyres
compromised by consumption of alco
when
blood alcohol
minor
road
late
at
night.
and the brake system of the car. In
mobile phones. Every year 300–400 p
content (BAC) is 10 mg/100 mL
bringing a car to rest its kinetic energy
about 1200 are seriously injured and o
If an unexpected event occurs (e.g. a person walks out onto the road) will
(0.01
g/100
mL,
0.01%
g/mL
or
must be dissipated in some way; ideally
It is important to ensure your tyres an
they be able to avoid a collision?
0.1 ing/L).
Consumption
of one
it generates heat
the brakes.
If the
good condition in order to protect you a
Fig. 4 Every year more then 300
Joe’s
response
time
might
be
expected
to
be
about
1.5
s;
during
that
brakes are applied
so hard
the the BAC by
standard
drinkthat
raises
people
are
killed
Fig. 2 Skid
marks
lefton
by our roads.
time
the
car
travels
33
m.
Once
the
brakes
are
applied
the
car
travels
a
wheels cannot turn then the car skids
emergency braking.
20 to 30 mg/100 mL, depending
further 37 m before it is brought to a stop. So the total stopping distance
and frictional energy is dissipated in
mainly on body mass – the
lower µ=0.7) is 70 m.
(assuming
the body mass the higher the BAC. In Europe the BAC legal limit for
drivers varies from zero (Czech Republic) to 80 mg/100 ml (Ireland and
Fig. 5
Brake testing in Deansgrange NCTS centre
UK) but is mostly 50 mg/100 ml or 0.05%.
In 2003, 36.5% of fatal road accidents in Ireland were alcohol related (cf.
report at http://www.rsa.ie/publication/publication/).
Driving requires constant attention and anticipation of possible hazards.
The ability to respond appropriately to unexpected events is seriously
compromised by consumption of alcohol and by distractions such as
mobile phones. Every year 300–400 people are killed on Irish roads,
about 1200 are seriously injured and over 10,000 suffer lesser injuries.
It is important to ensure your tyres and brakes are maintained and in
good condition in order to protect you and others on the road.
Find out more about the NCTS and car testing at
www.ncts.ie and at www.sta.ie. You can also find out
more about the SGS Group at www.ie.sgs.com.
Road Safety
Science
Every year some hundreds of people are killed on the roads
in Ireland – about one a day on average. Awareness campaigns
have not produced the desired change in driver behaviour.
An understanding of the factors that are common to many
road accidents and an understanding of the laws of motion
Syllabus
Reference
may
help increase
awareness of the dangers and encourage
young people to take greater care on the road whether as
Leaving Certifi
cate
Physicsor drivers.
pedestrians,
cyclists,
passengers
1.1. Linear motion
Reaction time (normal reaction time):
and
and
Science
Road S
The estimated stopping distance for
time of say 2 s and a coefficient of fricti
•
Drivers are advised to leave at least a ‘t
In three seconds a car travelling at 80
speed it would require a very fast re
coeffi
cient
friction (0.9)
to bring a ca
The stopping distance for a car travelling at 80
km/h
isoftypically
in the
50 m. These estimates do not take acc
range 50 to 125 m. At 120 km/h it is 90 to 240 which
m (depending
on
driver
is about 200–300 ms.
response, road conditions and the kind and condition of the tyres).
•
inflation
Three seconds should be regarded as Tyre
a minimum
time gap
The frictional force between two surf
between vehicles.
2.1. Newton’s
laws of
involved and on the force between th
The
average time
for motion
does not depend on the contact are
reaction to visual stimuli
case of a car, the force between the s
(with no choices) is
Junior180
Certifi
cate
distributed over the four wheels. If the
between
and 200
ms Science
weight of the car presses on a small a
(0.18–0.20
s). In–the
case
Section 3A1
Measurement
in science; 3A3 Force and moments; 3A5
the same but it is transferred through a
of auditory stimuli it is
Fig. 3 Adverse weather
can create
dangerousof
driving
conditions.
Get
two
pieces
wood
of
about
equal
Work and power; 3A6 Energy; 3A7 Energy conversion
frictional force is then more likely to ex
shorter (140–160 ms). When
size (e.g. 300 × 150 × 15 mm). Glue a layer of rubber
(e.g.
from and
an old
the rubber
together
bits of the ty
decisions are required the
the tyres and the road; when this happens the car cannot be steered and
car tube) to one face of one of the pieces; glue
half
asare
much
rubber
wider
tyres
desirable.
time required to make an
the stopping distance increases.
to the second piece. Then attach a hook to one
end
of ofeach
piece the area
appropriate response to a
Over-infl
ation
tyres reduces
) =wood.
½m v 2
Kinetic energy (Eof
visual stimulus increases
k
tyre is more likely to be abraded by
with age from about 370
Work done = frictional force × distance = F × s
steering more difficult.
Fig. 1 Scene of a crash – a common
ms, for 20–30 year olds, to
Place
blocks,
side
down, on
level path (concrete or tarmac).
Frictional force is equal
to the
the coeffi
cientrubber
of friction
multiplied
by a
the
occurence
on
Irish
roads
Higher speed reduces traction (grip o
On
completing
this
lesson,
students
should
be
more
about 440 ms for 60–70
each block in turn with large masses (e.g. 3,
4, 5 kg) and measure
weight of the car: F =Load
µ × mg
probability of aquaplaning in wet condi
year
olds. that:
aware
the
forceorder
it can
withstand
without
In the case of cars that
aremaximum
in good working
travelling
on a dry
road moving. Does the force vary
Other
factors
time are:
with(µ)the
sizefrom
of the
total
of each
block? Does it vary with
• cars
dothat
notaffect
stopreaction
the instant
the brakes are applied
the coefficient of friction
ranges
about
0.6 weight
(for ordinary
cars) loaded
to
Alcohol
about 0.95 (for the most
cars).
the expensive
size of the
rubber surface?
• the complexity of the situation.
In
• the stopping time comprises response time, brake activation time
In braking
Ek = F × s
o
• the illumination (dim situations require more time to more interpret).
In
each
case
divide
the
maximum
force
by
the
weight
(mg).
(This
is
the
and braking time
s
½ m v 2 = (µ × mg) × s
• a driver’s general level of alertness.
coefficient of friction.) What do you notice?
a
s = v 2/(2 × µ × g),
• the stopping distance is the total distance a car travelsTherefore
during
p
• normal but unexpected events (e.g. brake lights on the car ahead).
If
possible
measure
the
force
required
to
keep
each
block
moving
response, brake activation and braking
ti
where g is the acceleration due to gravity.
• unusual and unexpected events (e.g. car approaching on the wrong
steadily. Is the static friction greater than the dynamic friction? How
c
This shows that the braking distance (s) is proportional to the speed
• side
theofbraking
the road).distance is proportional to the square of the speed
s
does this apply to braking?
squared (v2); e.g. doubling the speed increases the stopping distance by
c
•• blood
level,coeffi
whichcient
is a factor
in about 33%
of road
accidents,
factor
of four. The mass of the car is not a factor. For an ordinary car
the alcohol
effective
of friction
varies
with
the type of acar,
the
(0
Measure people’s reaction time in situations requiring no choice and in
increases reaction time and impairs judgement.
travelling
at
100
km/hour
(27.8
m/s)
the
braking
distance
would
be
about
condition of the tyres (tread, inflation) and the condition of the road
0
Fig. 4 Every
year more
then 300
situations
requiring
two
(or more)
choices. (A simple
online
reaction
55
m;
this
does
not
take
account
of
the
driver’s
response
time.
Many(wet/dry,
studies have
shown that 95% of real response times of drivers are
s
people are killed on our roads.
slope)
timer
is
available
at
http://getyourwebsitehere.com/jswb/rttest01.html#)
in the range 1.5 to 2.5 seconds; the faster responses were recorded
2
In wet conditions the coefficient of friction is about 40% less and if
Remember
that
reactions
times
are
much
longer
in
the
case
of
when
drivers
were warned
that there
be alertness,
a hazard and
wereof vision
m
• the
response
time varies
withmight
driver
clarity
the (light
wheels lock it may be as low as 0.1, increasing the braking
therefore in a state of readiness.
the body mass the higher the BAC. In
unexpected events.
distance dramatically.
level, glare)
drivers varies from zero (Czech Repub
UK) but is mostly 50 mg/100 ml or 0.05
Two examples
• consumption of alcohol impairs driving skills (appropriate response,
Student Activity
Learning Outcomes
Speed and braking
judgement, steering ability,
visual function).
distance
True/False Questions
In 2003, 36.5% of fatal road accidents in
Consider the following two situations:
report at http://www.rsa.ie/publication/
The stopping distance for a car depends
1. Joe is an average driver, alert and driving at 80 km/h (22.2 m/s) on a
on driver reaction time, the speed of
Driving
requires constant
and
(a) Stopping distance is not the same as braking
distance.
T attention
F
main road in daylight.
the car, the slope and condition of the
The ability to respond appropriately to
2.
Pat
is
an
average
driver,
tired
and
driving
at
80
km/h
on
a
poorly
lit
road surface, the condition of the tyres
compromised by consumption of alco
(b) Most drivers can respond to unexpected situations
minor road late at night.
and the brake system of the car. In
mobile phones. Every T
year
in less than a second.
F 300–400 p
bringing a car to rest its kinetic energy
about 1200 are seriously injured and o
If an unexpected event occurs (e.g. a person walks out onto the road) will
be dissipated in some way; ideally
Work equals force multiplied by distance. It is important to ensure
T your
F tyres an
be able to avoid(c)
a collision?
• In braking a car’s kineticmust
energy
must be dissipated (usually asthey
heat).
it generates heat in the brakes. If the
good condition in order to protect you a
Joe’s
response
time
might
be
expected
to
be
about
1.5
s;
during
that
brakes are applied so hard that the
(d) Kinetic energy is proportional to speed.
T F
•Fig. the
kinetic
car;
2 Skid marks
left byenergy = the work done in stopping the
time the car travels 33 m. Once the brakes are applied the car travels a
wheels cannot turn then the car skids
emergency
× s = µ × mg ×and
s. frictional energy is dissipated in
Ek = Fbraking.
further 37 m before it(e)
is brought
to
a
stop.
So
the
total
stopping
distance
Over 10,000 people are injured on Irish road each year.
T F
(assuming µ=0.7) is 70 m.
General Learning Points
• Braking distance is proportional to the speed squared.
(f) Friction between car wheels and the road is essential for
accelerating; braking and turning or steering.
T F
(g) Over inflation of tyres increases the braking distance.
T F
(h) If the wheels lock (are unable to rotate) the braking
distance is reduced.
T F
(i) The braking distance of a car is unaffected by rain.
T F
• Under-inflation of tyres impairs steering.
(j) Aquaplaning occurs in wet conditions and does
not depend on speed.
T F
• Response time to unexpected situations is typically in the range 1.5
to 2.5 seconds.
(k) A car travelling at 120 km/h covers 33 m in one second.
T F
• The effective coefficient of friction is not the same for all cars; the
more expensive cars generally have wider tyres which can transfer
more force without mechanical failure (serious abrasion).
• The coefficient of friction decreases by about 40% in wet conditions
(assuming the tyres are in good condition).
• Over-inflation of tyres reduces stopping distance.
• Dimly lit situations require more processing time.
Check your answers to these questions on www.sta.ie
and
Science
Road Safety
IN
AND
SCIENCE TECHNOLOGY
SECOND
EDITION
EDITION
ACTION THIRD
d Safety
Every year some hundreds of people are killed on the roads
in Ireland – about one a day on average. Awareness campaigns
have not produced the desired change in driver behaviour.
An understanding of the factors that are common to many
road accidents and an understanding of the laws of motion
Examination
Questions
may help increase awareness
of the dangers and encourage
young people to take greater care on the road whether as
2006
Leaving
Certifi
cate Ordinary
Level
pedestrians,
cyclists,
passengers
or drivers.
The estimated stopping distance for
time of say 2 s and a coefficient of fricti
Drivers are advised to leave at least a ‘t
In three seconds a car travelling at 80
speed it would require a very fast re
coeffi
of and
friction
(0.9)for
to bring a ca
The water content of the body tissues is about 58%
forcient
men
49%
50 m. These estimates do not take acc
women and men are generally heavier than women
(76
kg
vs.
62
kg
for
which is about 200–300 ms.
A person pushed a car a distance of 15 m with a force of 500 N. Calculate
Reaction
time
(normal
reaction
the
work done
by the
person. (W
= Fs) time):
The average time for
reaction to visual stimuli
2002
Leaving
Certifi
(with no
choices)
is cate Ordinary Level
between 180 and 200 ms
a)
What
is
friction?
(0.18–0.20 s). In the case
of auditory stimuli it is
b)
A car of mass 800 kg is travelling at 10 m s-1.
shorter (140–160 ms). When
decisions are required the
What is its kinetic energy? (Ek = ½mv 2 )
time required to make an
appropriate response to a
visual stimulus
2005
Juniorincreases
Certificate Higher Level
with age from about 370
Friction
is an
example
a force.
1 SceneIt
of acts
a crashin
– amany
commonways.
ms, for 20–30
year
olds, to of Fig.
occurence on Irish roads
about 440
ms for
60–70
driven,
friction
can
be both
useful and not useful.
year olds.
25 year olds). These two factors increase the effects of alcohol
consumption on BAC of young women by about Tyre
45%. inflation
The coefficient of friction of between rubber and The
ice frictional
is aboutforce
0.25between
if dry two surf
involved and on the force between th
and less than 0.2 if wet; treating the surface with
grit
increases
the
does not depend on the contact are
friction to about 0.4. However loose grit reduces the
coeffi
friction
case
of a cient
car, theofforce
between the s
distributed over the four wheels. If the
on dry, ice-free roads.
Fig. 3 Adverse weather can create dangerous driving conditions.
the tyres and the road; when this happens the car cannot be steered and
the stopping distance increases.
Biographical Notes
Kinetic energy (Ek ) = ½m v 2
Work done = frictional force × distance = F × s
weight of the car presses on a small a
the same but it is transferred through a
frictional force is then more likely to ex
the rubber together and bits of the ty
wider tyres are desirable.
Over-inflation of tyres reduces the area
tyre is more likely to be abraded by
steering more difficult.
Aristotle (384–322 BC)
When aFrictional
car is force is equal
to the coefficient of friction multiplied by the
Higher speed reduces traction (grip o
weight of the car: F Aristotle
= µ × mg entered Plato’s academy at the age of eighteen and studied
probability of aquaplaning in wet condi
there for twenty years. Some years after Plato’s death Aristotle set up
In the case of cars that are in good working order travelling on a dry road
Give
two
examples
of
when
friction
is
useful
when
a
car
is
driven.
Other factors that affect reaction time are:
his own
academy
which
the
Lyceum.
the coefficient of friction
(µ) ranges
from about
0.6he
(forcalled
ordinary
cars)
to
Alcohol
about 0.95 (for the most expensive cars).
Give
examples
when friction is not useful when a car is driven.
• thetwo
complexity
of theof
situation.
In
The Greek approach to science was theoretical and often quite
In braking
Ek = F × s
o
• the illumination (dim situations require more time to more interpret).
speculative; it was not customary to carry out experiments to
s
½ m v 2 = (µ × mg) × s
2004
Junior
Certifi
Higher Level
verify theories.
• a driver’s
general
level ofcate
alertness.
a
Therefore
s = v 2/(2 × µ × g),
A
was
30 m/s
the brakes
were
applied. The car
p
• car
normal
buttravelling
unexpectedatevents
(e.g.when
brake lights
on the car
ahead).
Aristotle
wrote
on
all
subjects
from
politics
and
ethics
to
logic
and
ti
g is the acceleration due to gravity.
came to rest in 12 seconds. The table gives the velocity of the carwhere
at two
• unusual and unexpected events (e.g. car approaching on the wrong
science. He proposed that the speed of falling bodies was proportional
c
second
This shows that the braking distance (s) is proportional to the speed
side ofintervals
the road). during this time.
s
to their weight and that the speed of each body did not change as it fell.
squared (v2); e.g. doubling the speed increases the stopping distance by
c
firstofsight
might
appear
• blood alcohol level, which is a factor in about 33% of road accidents,
a factor of four. TheAt
mass
the carthis
is not
a factor.
For anreasonable.
ordinary car However it is easy to show
(0
Velocity
(m/s)
20
15
10
5
0
increases
reaction time and30
impairs25
judgement.
that falling
bodies
do not
fall at
a constant
travelling at 100 km/hour
(27.8 m/s)
the braking
distance
would
be about speed – they accelerate.
0
Many studies have shown that 95% of real response times of drivers are
Time (s)
0
2
4
6
8
in the range 1.5 to 2.5 seconds; the faster responses were recorded
when drivers were warned that there might be a hazard and were
therefore
in a stateon
of readiness.
Draw
a graph,
graph paper, of velocity against time.
55 m; this does not take account of the driver’s response time.
Fig. 4 Every year more then 300
s
our roads.
Strato, who was one of Aristotle’s successors aspeople
headareofkilled
theonLyceum,
2
In wet conditions the coefficient of friction is about 40% less and if
observed
that
as
rain
ran
off
a
roof
the
distance
between
the
drops
m
the wheels lock it may be as low as 0.1, increasing the braking
increased as they fell. He deduced that theirthe
speed
could
bethe BAC. In
body mass
thenot
higher
distance dramatically.
drivers
varies fromthat
zero the
(Czech Repub
constant – they were in fact accelerating. He also
observed
UK) but is mostly 50 mg/100 ml or 0.05
Two examples
Put velocity on the y-axis. Use
the graph
find
Speed
and to
braking
impact of a stone on the ground varied with the height
from which it was
In 2003, 36.5% of fatal road accidents in
two situations:
dropped;
therefore it did not have a constant speed.
distance
(i) the time taken for the velocity
of the car to reduce to 12.5 m/sConsider the following
10
12
report at http://www.rsa.ie/publication/
The stopping distance for a car depends
1. Joe is an average driver, alert and driving at 80 km/h (22.2 m/s) on a
Despite this insight Aristotle’s views on motion
dominated natural
(ii) the velocity of the car 3on
seconds
after the
were
driver reaction
time,brakes
the speed
of applied main road in daylight.
Driving requires constant attention and
philosophy for nearly 2000 years – until the time The
Galileo
Newton.
the car, the slope and condition of the
abilityand
to respond
appropriately to
2. Pat is an average driver, tired and driving at 80 km/h on a poorly lit
(iii) the acceleration of the car.
road surface, the condition of the tyres
compromised by consumption of alco
For further
Fig. 2 Skid marks left by
emergency braking.
and the brake system of the car. In
bringing a car to rest its kinetic energy
examples
of pastinpaper
questions
must be dissipated
some way;
ideally
it generates
heat in the brakes. If the
check
www.sta.ie
brakes are applied so hard that the
wheels cannot turn then the car skids
and frictional energy is dissipated in
minor road late at night.
mobile phones. Every year 300–400 p
Find out more about other famous scientists
about on
1200www.sta.ie
are seriously injured and o
If an unexpected event occurs (e.g. a person walks out onto the road) will
they be able to avoid a collision?
Joe’s response time might be expected to be about 1.5 s; during that
time the car travels 33 m. Once the brakes are applied the car travels a
further 37 m before it is brought to a stop. So the total stopping distance
(assuming µ=0.7) is 70 m.
Did You Know?
The effects of blood alcohol content (BAC)
No. of
drinks
BAC
(mg/100 mL)
Effects
1
20 to 30
noticeable effects (on perception etc.)
5
100 to 150
intoxication
24
240 to 360
unconsciousness
36
360 to 480
coma or death
It is important to ensure your tyres an
good condition in order to protect you a
Revise The Terms
Can you recall the meaning of these terms? Reviewing the
terminology is a powerful aid for recall and retention.
Velocity; energy; kinetic energy; mass; force; work; the principle of
conservation of energy; newton (N); friction; coefficient of friction; static
friction; dynamic friction, illumination; coefficient of friction; braking
distance; response time; stopping distance; brake engagement time;
abrade; aquaplaning; standard drink
Check the Glossary of Terms for this lesson
at www.sta.ie