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S3 FORCES
THE PHYSICS OF CAR SAFETY
© C. Rossi, Dumfries High School
S3 Forces: The Physics of Car Safety
Lesson 1,2 – Forces on cars
1. What do we already know about the effects of a force?
2. How are these effects important in ensuring car users
can travel safely?
3. Can you recall from S2 what is meant by “balanced”
and “unbalanced forces”?
4. How is the speed of a car is controlled by the sizes of
the drive force and the drag forces?
5. How does the time taken to stop a car affect the size of
the braking force?
6. Why are “crumple zones” used in car design?
2
What is force?
Think about cars - in your jotter try to:




3
give 5 examples of where you might see a force.
E.g. Opening a door, pressing the accelerator or
brakes, turning a spanner, using a jack, turning the
steering
Write a 3 things a force can do to a car.
Change the speed, direction or shape of the car
Newton’s First Law
An object will remain stationary or move at a constant speed
in one direction unless an unbalanced force acts upon it.
Balanced forces
When there are two or more forces acting
on an object and they add up to a total
force of zero, we say the forces are
balanced.
Unbalanced forces
When the total force is NOT zero we say
there is an unbalanced force. This is equal
to the force “left over” and will cause the
object accelerate or change direction or
shape.
4
What happens to a car if forces are balanced?
Having EQUAL AND OPPOSITE forces acting on a car
is effectively the same as having NO force acting.
drag
drive
The car moves at constant speed.
Newton’s 1st Law: An object will remain stationary or move
at a constant speed in one direction unless an
unbalanced force acts upon it.
What happens to a car if forces aren’t balanced?
For
example
when
a driver
What
happens
when
there
who
is travellingforces
at
are unbalanced
constant
accelerates
acting on speed
an object?
or puts on the brakes.
Accelerate:
drive
Brake:
drag
Backuntil
arrowdrag
twice
Speed
Speed decreases until
drag increases
to see
again…
drive.
matches drive again.grows to match
Find the UNBALANCED FORCE
In each of the following calculate the unbalanced force and its direction.
(a)
30N
(d)
60N
50N
(b)
10N
10N
(e)
60N
50N
20N
10N
50N
(c)
50N
7
45N
(f)
10N
90N
60N
110N
Find the UNBALANCED FORCE - answers
(a)
(d)
20N
(b)
20N
(e)
50N
(c)
5N
8
(f)
30N
20N
FORCES acting on a CAR - 1
Reaction (push back from ground)
Is the car…
Stationary
No Drive
Force
No Drag
Force
Moving with
constant speed
Accelerating
Weight
Decelerating
Unbalanced force = zero (how do you know?) and the car is
stationary. If it was moving there would be a drag force.
9

FORCES acting on a CAR - 2
Is the car…
Stationary
Moving with
constant speed
Accelerating
Decelerating
Unbalanced force = zero
The car remains at the same speed, in the same
direction
10

FORCES acting on a CAR - 3
Is the car…
Stationary
Moving with
constant speed
Accelerating
Decelerating
Unbalanced force is NOT zero,
so…
The car accelerates in the direction of the unbalanced force
(forwards)
11

FORCES acting on a CAR - 4
Is the car…
Stationary
Moving with
constant speed
Accelerating
Decelerating
Unbalanced force is not zero,
The car slows down (decelerates).
12

FORCES acting on a CAR - 5
Road
force
Drive
force
Drag
Weight
•Road force and weight are always equal
•If the drive force equals the drag – steady speed
•If the drive force is bigger than drag – car accelerates
•If drag is bigger than the drive force – car decelerates
13
FORCES acting on a CAR - POSTER
1. Draw four diagrams of a car (same car in each case – nothing too elaborate).
•Car
•Car
•Car
•Car
one is at rest.
two is moving at constant speed forwards.
three is accelerating.
four is decelerating.
Each diagram should show all of the labelled forces acting on the cars.
2. Explain why all cars have a top speed.
Extra if you have time:
A car which was moving at constant speed drives off a cliff.
3. Explain – in detail - what happens next in terms of the motion of the car and
all of the forces involved. Include diagrams.
Stopping cars safely:
Let us now try to figure out how stop a car safely.
What do we need to do to stop a car?
Apply the brakes!!
The FORCE of the brakes SLOWS the car.
Does the SIZE of the force matter? – let’s see…..
Stopping cars safely:
Both cars are moving at constant velocity.
No acceleration.
Watch
happens when the drivers brake….
Forces
arewhat
BALANCED
Stopping cars safely:
The drivers now apply the
BRAKES.
But the far away driver
brakes harder!
But which car stops
QUICKEST?
Watch again----
Stopping cars safely:
Which car stops
QUICKEST?
BIG force  SMALL stopping time
SMALL force  BIG stopping time
The car with the
BIGGEST braking
force stops in a
SHORTER time.
Force and time.

When a _____is used to slow down a car it is useful
to remember that
•
A BIG force will slow the car in a ______time, and
•
A SMALL force will slow the car in a ______time
Missing words: LONG, force, SHORT
19
Force and time.

When a force is used to slow down a car it is useful
to remember that
•
A BIG force will slow the car in a SHORT time, and
•
A SMALL force will slow the car in a LONG time
•
How are these ideas useful in other car situations?
What if the car COLLIDES to a stop?
•
20
CRUMPLE ZONES
New car with crumple zone.
Is there any
difference
between
the two
collisions?
Old car without crumple zone.
Compare the impacts in slow motion…
The
carhow
withlong
theeach
crumple
Watch
crashzone
lasts
takes longer to stop
Let’s now think about forces…
Smaller
This
car
takes longer
forces
break
to stop.
less
bones!
LONG TIME SMALL
FORCE
Assuming the cars have the same mass,
Which car iszones
safest tosave
travel in?
Crumple
lives!!
Which car is stopped by the biggest force?
Bitesize Crumple Zones in Cars
Bigger
This
car
stops –
forces
quickly
DANGER!
SHORT TIME LARGE
FORCE
Which car is the safest?
How should a car be
designed to cope safely
with collisions?
The Road Transport Laboratory
suggests the most important
safety features are
• The driver and passenger should be able to experience
the car slowing down comfortably.
• The inside of the car should not be squashed.
• Most of the energy of the collision should be absorbed
by the car body and not the driver and passengers.
Crumple Zones

•
•
•
When a c_____ z____ is part of the bodywork of
the car that is designed to crumple during a c_____.
This ensures that the car and occupants are stopped
in a relatively L____ time.
That then also means the FORCE used to slow the
car is relatively S______.
Small forces will not tend to h____ the occupants of
the car as m_____.
Missing words:
crash, much, LONG, harm, crumple zone, SMALL
26
Crumple Zones

•
•
•
27
When a crumple zone is part of the bodywork of the
car that is designed to crumple during a crash.
This ensures that the car and occupants are stopped
in a relatively LONG time.
That then also means the FORCE used to slow the
car is relatively SMALL.
Small forces will not tend to harm the occupants of
the car as much.
Yourhave
You
group’s
probably
task isseen
to take
TVan
shows
ordinary
where an
physics
trolley
ordinary
and car
fit itiswith
fitted
a bumper
with an
amazing
or
crumple
body
zone
kit and
to make
a fancy
it safer
stereo
in a
before being
collision.
Yougiven
can make
a stunning
it stylish,
paintbut
job.
safety
is more important!
You will be given a choice of
materials plus card and sticky
tape.
Your design must be able to be fixed
to the front of the test trolley with BluTac.
It should not add more than
2 cm to the length of your
trolley.
It must not have a large effect on the
performance of the trolley.
Your design will be tested. The test will measure the force acting on the
vehicle during a head-on collision.
S3 Forces: The Physics of Car Safety
Lesson 3,4 – Seat belts and air bags
What can we say about the speed of a car if the forces
acting on it are balanced?
Why are seat belts are used in cars?
If the time taken to bring the driver or passenger to a
stop is increased, how will this affect the force used to
slow them?
Why are airbags are used in cars?
29
Crash test dummies
The Physics of Car Safety
Car designers safety test their
designs using real cars and life size
models known as crash test
dummies.
Crash test dummy video
Crash test dummies cartoon (20min)
Car Safety info
Let’s examine in detail how forces
play a huge part in safely stopping a
driver:
30
Forces on a passenger during a crash
The car is about to brake suddenly.
The dummy has NO seatbelt.
The car stops – but the dummy continues to move as
The
there is no unbalanced force
to dummy
stop it. hits the dashboard and
stops VERY QUICKLY.
SHORT time – LARGE FORCE!
NOT SAFE!!
Forces on a passenger during a crash
Now let’s see the difference when
an AIRBAG is used
With an airbag, the time to
stop the dummy is BIGGER.
LONG time – SMALL force
SAFER!!
Forces on a passenger during a crash
How do seatbelts protect the
passenger?
When the car stops, the seatbelt
provides an unbalanced force
to stop the dummy SAFELY!
Stopping passengers safelyseat belts and air bags.
34

When the car stops, the seatbelt provides an
________ force that prevents the driver or
passenger from hitting the ___________

With an airbag, the time to stop the driver or
passenger is __________.

A __________ force is needed so there is less
chance of ________ the driver or passenger.
Missing words:
smaller, dashboard, hurting, unbalanced, bigger
Stopping passengers safelyseat belts and air bags.
35

When the car stops, the seatbelt provides an
unbalanced force that prevents the driver or
passenger from hitting the dashboard.

With an airbag, the time to stop the driver or
passenger is bigger.

A smaller force is needed so there is less
chance of hurting the driver or passenger.
Crash test dummy investigation

Set up the ramp and low wall at the bottom of the ramp for the car to
crash into.
clay person
ramp on
stand





36
dynamics
trolley
low wall
Form three clay people of varying masses to represent an adult, a
teenager and a baby.
Place one of the people in the car and let it roll down the ramp and
crash into the wall.
Measure the distance the person flies out of the car over the wall and
record the distance in a data table.
Complete three trials with each clay person released from the same
height on the slope. Record your results.
Finally repeat the experiment using an elastic to represent a seatbelt
that restrains the person from flying off the trolley.
Crash test dummy investigation –
Results
Trial
Baby
Teen
Distance clay person
flew (cm
1
2
3
1
2
3
Adult
1
2
3
37
Observations
Crash test dummy investigation –
Analysis
1. Describe Newton’s first law of motion.
2. What factors, besides mass, could have affected
how far each passenger flew out of the car?
3. Why did the people fly out of the car?
4. What variables were held constant in this
investigation?
5. What could be done to the car to make the people
fly out farther?
6. When the seat belts were put on the passengers
what happened to the people when the car hit the
low wall and stopped?
38
Crash test dummy investigation –
Answers
1. An object will remain stationary or move at a constant
speed in one direction unless an unbalanced force acts
upon it.
2. Speed of the car, friction between the person and
ground, air resistance
3. They were unrestrained so did not have an unbalanced
force acting on them to stop them with the trolley.
4. Speed of the car – release height on ramp, friction
5. Release the car from higher on the ramp, so making
the crash speed higher.
6. Because there was an unbalanced force from the
seatbelt to person, the person did not fly off the trolley.
39
S3 Forces: The Physics of Car Safety
Lesson 5,6 – Safe stopping distance
1. What are “braking distance, thinking distance and
stopping distance of a car”?
2. What can affect the braking distance, thinking
distance and stopping distance of a car?
3. How does stopping distance depends on driver
reaction time and car speed?
4. How does braking distance depend on vehicle speed,
vehicle mass, and road surface?
40
Safe stopping distance


Stopping distance
depends on the
speed of the car
That is why we
have speed limits the more likely it is
for a driver to have
to stop, the lower
the speed limit.







41
Guess the car
stopping distances
at each of these
speeds:
20 mph -12
?m
30 mph -23
?m
40 mph -36
?m
50 mph -53
?m
60 mph -73
?m
70 mph -96
?m
Safe stopping distance





42
Stopping distance is the
distance travelled by the
car from when the driver
notices the hazard to
when the car stops.
Stopping distances are
spilt into two sections:
thinking distance: the
distance gone as reacts to
whatever is making
him/her stop
braking distance: the
distance travelled once
the brakes are pressed
Stopping distance =
thinking distance +
braking distance
Stopping distances
How long does it take a moving vehicle to stop?
STOPPING DISTANCES ANIMATION
Stops
TheBraking
Thinking
stopping
distance
distance
distance
is is
thethe
is the
distance
sum
distance
of the
a car
thinking
a car
travels
travels
distance
before
and
whilst
thethe
braking
the
brakes
brakes
distance.
areare
applied.
being applied.
Presses brake
Braking
distance
Sees cat
Thinking
distance
Stopping distance
Stopping distance = thinking distance + braking distance
FACTORS AFFECTING STOPPING
DISTANCE
ROSPA CRASH SIMULATOR
Do you know your speed limits?
?
Match up the words with their definitions.
45
Stopping distance
One of forces the road exerts on
the tyres as the car is stopping.
Friction
The distance a car travels whilst
it is braking.
Thinking distance
The distance a car travels before
the brakes are applied.
Braking distance
The sum of thinking distance
and the braking distance.
What factors affect braking
and thinking distance?
Thinking distance
Braking distance
Speed of car
Speed of car
Drugs and alcohol
Road conditions
Medication
Tiredness
Condition of tyres
Medication
Condition of brakes
Tiredness
Drugs and alcohol
Speed of car
Condition of tyres
46
Condition of brakes
Road conditions
Stopping a car depends on…
Tiredness
Too many
drugs
Thinking
distance
47
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
Too much
alcohol
Braking
distance
Driving
too fast
Why can an icy road increase the
stopping distance of a car?


48
When you brake the car
goes into a skid as the
friction between the road
and tyres is very small..…
..so there is hardly any
unbalanced force to slow
the car – so it goes
further before stopping
Why do bald tyres increase the
stopping distance of a car?
• When you brake the car
goes into a skid as the
friction between the road
and tyres is very small,
especially in wet
conditions..…
• ..so there is hardly any
unbalanced force to slow
the car – so it goes
further before stopping
49
Question : A car is moving along an open
road. Suddenly, a sheep walks into the road.
a) What do we call the distance the car travels before the driver puts their
foot on the brakes?
Thinking distance
b) Name one factor that could increase the distance the car travels in this
time.
Medication, drugs/alcohol, speed of car, tiredness
c) The braking distance is 35m for the car. If the stopping distance is
50m, how far did the car travel before the driver put their foot on the
brakes?
Thinking distance
= Stopping distance – braking distance
= 50m – 35m
= 15m
50
Measuring your reaction time.
Do this:
Find your reaction time for a dry road at 30
mph.
What are the thinking and braking distances at
this speed?
Record your results.
Without changing any other controls, turn the
road conditions dial to wet.
What are the thinking and braking distances for
these conditions?
Repeat for icy conditions.
Can you explain these results?
Try to use the correct science words.
51
Measuring your reaction time results
You could use a table like this to record reaction (thinking) time:
Record these at different speeds (thinking time won’t change – you will
just adjust the speed dial each time).
Speed
(mph)
Thinking time
(milliseconds)
Thinking
distance
(metres)
Braking
distance
(metres)
Stopping
distance
(metres)
20
30
Stopping distance = thinking distance + braking distance
You could use a table like this for the road conditions task.
Road
conditions
Thinking time
(milliseconds)
Thinking
distance
(metres)
Braking
distance
(metres)
Stopping
distance
(metres)
dry
wet
icy
Graphs and bar charts are good ways of presenting information like this.
Measuring your reaction time.
Do this:
Find your reaction time for a dry road at 70
mph.
What are the thinking and braking distances at
this speed?
Record your results.
There is a gap beneath the reaction timer that
you can fit a CD into.
Try to remove a CD from the gap and replace it
with another whilst measuring your reaction
time.
Record your reaction time, thinking and braking
distances.
Think of other distractions when driving. Find
their effects on your reaction time.
REACTION TIME JAVA APPLET
53
Investigating stopping distance
Cars and carpet
mask fitted to car
The idea of this
experiment is to enable
you to understand the
connection between
speed and stopping
distance of a toy car.
o
o
o
o
54
toy car
stopping distance
Measure the mass of the car
Allow the car to run down the ramp and record:
o
o
light gate
(a) the speed (mask length / time
(b) the distance that the car travels on the carpet before
stopping
Repeat this twice so that you have three pairs of values of
speed and distance.
Record your results in a table.
Investigating stopping distance
Cars and carpet RESULTS
mass of
car
(g)
mask length
(cm)
time to pass
light gate
(s)
speed = mask
length /time
(cm/s)
stopping
distance
(cm)
1. What effect did increasing the speed have on the
stopping distance of the car?
2. What does this result lead you to think about real cars
and car safety?
3. Can you think of another experiment you could do that
would discover what else might affect the stopping
distance of the car? If you have time try the experiment .
55
What can increase
stopping distances?

Braking distance is affected by:
 The c___ ……………. .(worn brakes, extra weight, bald tyres etc)
 The r____ .…………… (poor surface, spilt oil etc)
 The w_______ ………..(wet, icy - anything that reduces friction)

Thinking distance is affected by:
 The driver's r________time.
 Your reaction time can be affected by:
 d__________in the car,
 distractions outside the car (not just distractions – s_______reflected into
eyes can affect the time interval between the braking occurrence
happening and the driver reacting to it),
 a___________,
 m______________,
 a___ - your thinking processes slow as you approach old age
Missing words:
medication, distractions, road, age, reaction, car, sunlight weather, alcohol,
56
What can increase
stopping distances?
57

Braking distance is affected by:
 The car ……………. .(worn brakes, extra weight, bald tyres etc)
 The road .…………… (poor surface, spilt oil etc)
 The weather ………..(wet, icy - anything that reduces friction)

Thinking distance is affected by:
 The driver's reaction time.
 Your reaction time can be affected by:
 distractions in the car,
 distractions outside the car (not just distractions - sunlight
reflected into eyes can affect the time interval between the
braking occurrence happening and the driver reacting to it),
 alcohol,
 medication,
 age - your thinking processes slow as you approach old age