1. THE PHYSICS OF CAR SAFETY
S3 FORCES
THE PHYSICS OF CAR SAFETY

2. S3 Forces: The Physics of Car Safety Lesson 1,2 – Forces on cars
What do we already know about the effects of a force?
How are these effects important in ensuring car users can travel safely?
Can you recall from S2 what is meant by “balanced” and “unbalanced forces”?
How is the speed of a car is controlled by the sizes of the drive force and the drag forces?
How does the time taken to stop a car affect the size of the braking force?
Why are “crumple zones” used in car design?

3. What is force? Think about cars - in your jotter try to:
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

4. 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.

5. 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.
The car moves at constant speed.
drag
drive
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.

6. What happens to a car if forces aren’t balanced?
For example when a driver who is travelling at constant speed accelerates or puts on the brakes.
What happens when there are unbalanced forces acting on an object?
Accelerate:
Brake:
drive
drag
Speed decreases until drag
matches drive again.
Speed increases until drag
grows to match drive.
Back arrow twice
to see again…

7. Find the UNBALANCED FORCE
In each of the following calculate the unbalanced force and its direction.
30N
50N
(a)
10N
60N
(b)
45N
(c)
(d)
(e)
20N
110N
(f)
90N

8. Find the UNBALANCED FORCE - answers

9.  FORCES acting on a CAR - 1 Is the car… Stationary Moving with
Reaction (push back from ground)
Is the car… 
Stationary
Moving with
constant speed
No Drive
Force
No Drag
Force
Accelerating
Decelerating
Weight
Unbalanced force = zero (how do you know?) and the car is stationary.
If it was moving there would be a drag force.

10.  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

11.  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)

12.  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).

13. 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

14. FORCES acting on a CAR - POSTER
1. Draw four diagrams of a car (same car in each case – nothing too elaborate).
Car one is at rest.
Car two is moving at constant speed forwards.
Car three is accelerating.
Car 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.

15. 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…..

16. Stopping cars safely: Both cars are moving at constant velocity.
No acceleration.
Forces are BALANCED
Watch what happens when the drivers brake….

17. Stopping cars safely: The drivers now apply the BRAKES.
But the far away driver brakes harder!
But which car stops QUICKEST?
Watch again----

18. BIG force  SMALL stopping time SMALL force  BIG stopping time
Stopping cars safely:
Which car stops QUICKEST?
The car with the BIGGEST braking force stops in a SHORTER time.
BIG force  SMALL stopping time
SMALL force  BIG stopping time

19. Missing words: LONG, force, SHORT
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

20. 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?

21. CRUMPLE ZONES Is there any difference between the two collisions?
New car with crumple zone.
Is there any difference between
the two collisions?
Old car without crumple zone.

22. The car with the crumple zone takes longer to stop
Compare the impacts in slow motion…
The car with the crumple zone
takes longer to stop
Watch how long each crash lasts

23. Crumple zones save lives!! SHORT TIME LARGE FORCE
Let’s now think about forces…
Smaller forces break less bones!
This car takes longer to stop.
LONG TIME SMALL FORCE
Assuming the cars have the same mass,
Which car is stopped by the biggest force?
Crumple zones save lives!!
Bitesize Crumple Zones in Cars
Which car is safest to travel in?
This car stops quickly
Bigger forces – DANGER!
SHORT TIME LARGE FORCE

24. Which car is the safest?

25. 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.

26. crash, much, LONG, harm, crumple zone, SMALL
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

27. Crumple Zones
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.

28. You will be given a choice of materials plus card and sticky tape.
Your group’s task is to take an ordinary physics trolley and fit it with a bumper or crumple zone to make it safer in a collision. You can make it stylish, but safety is more important!
You have probably seen TV shows where an ordinary car is fitted with an amazing body kit and a fancy stereo before being given a stunning paint job.
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 Blu-Tac.
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.

29. 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?

30. 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:

31. Forces on a passenger during a crash
The car is about to brake suddenly.
The dummy has NO seatbelt.
LARGE FORCE
The car stops – but the dummy continues to move as
there is no unbalanced force to stop it.
The dummy hits the dashboard and stops VERY QUICKLY.
SHORT time – LARGE FORCE!
NOT SAFE!!

32. Forces on a passenger during a crash
Now let’s see the difference when an AIRBAG is used
SMALL FORCE
With an airbag, the time to
stop the dummy is BIGGER.
LONG time – SMALL force
SAFER!!

33. Forces on a passenger during a crash
How do seatbelts protect the passenger?
SEATBELT FORCE
When the car stops, the seatbelt
provides an unbalanced force
to stop the dummy SAFELY!

34. Stopping passengers safely- seat belts and air bags.
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

35. Stopping passengers safely- seat belts and air bags.
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.

36. Crash test dummy investigation
Set up the ramp and low wall at the bottom of the ramp for the car to crash into.
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.
clay person
dynamics trolley
ramp on stand
low wall

37. Crash test dummy investigation – Results
Trial
Distance clay person flew (cm
Observations
Baby 1 2 3
Teen
Adult

38. Crash test dummy investigation – Analysis
Describe Newton’s first law of motion.
What factors, besides mass, could have affected how far each passenger flew out of the car?
Why did the people fly out of the car?
What variables were held constant in this investigation?
What could be done to the car to make the people fly out farther?
When the seat belts were put on the passengers what happened to the people when the car hit the low wall and stopped?

39. Crash test dummy investigation – Answers
An object will remain stationary or move at a constant speed in one direction unless an unbalanced force acts upon it.
Speed of the car, friction between the person and ground, air resistance
They were unrestrained so did not have an unbalanced force acting on them to stop them with the trolley.
Speed of the car – release height on ramp, friction
Release the car from higher on the ramp, so making the crash speed higher.
Because there was an unbalanced force from the seatbelt to person, the person did not fly off the trolley.

40. S3 Forces: The Physics of Car Safety Lesson 5,6 – Safe stopping distance
What are “braking distance, thinking distance and stopping distance of a car”?
What can affect the braking distance, thinking distance and stopping distance of a car?
How does stopping distance depends on driver reaction time and car speed?
How does braking distance depend on vehicle speed, vehicle mass, and road surface?

41. Safe stopping distance
Guess the car stopping distances at each of these speeds:
20 mph - ?
30 mph - ?
40 mph - ?
50 mph - ?
60 mph - ?
70 mph - ?
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.
12 m
23 m
36 m
53 m
73 m
96 m

42. Safe stopping distance
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

43. Stopping distance = thinking distance + braking distance
Stopping distances
How long does it take a moving vehicle to stop?
STOPPING DISTANCES ANIMATION
Stops
Presses brake
Sees cat
Thinking distance is the distance a car travels before the brakes are applied.
The stopping distance is the sum of the thinking distance and the braking distance.
Braking distance is the distance a car travels whilst the brakes are being applied.
Braking distance
Thinking distance
Stopping distance
Stopping distance = thinking distance + braking distance
FACTORS AFFECTING STOPPING DISTANCE
ROSPA CRASH SIMULATOR

44. Do you know your speed limits?

45. Match up the words with their definitions.
Stopping distance
Friction
Thinking distance
Braking distance
One of forces the road exerts on the tyres as the car is stopping.
The distance a car travels whilst it is braking.
The distance a car travels before the brakes are applied.
The sum of thinking distance and the braking distance.

46. What factors affect braking and thinking distance?
Braking distance
Speed of car
Speed of car
Drugs and alcohol
Road conditions
Condition of tyres
Tiredness
Medication
Condition of brakes
Medication
Tiredness
Condition of brakes
Drugs and alcohol
Speed of car
Road conditions
Condition of tyres

47. Stopping a car depends on…
Tiredness
Too much alcohol
Thinking distance
Too many drugs
Poor visibility
Wet roads
Icy roads
Braking distance
Tyres/brakes worn out
Driving too fast

48. Why can an icy road 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..…
..so there is hardly any unbalanced force to slow the car – so it goes further before stopping

49. 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

50. Medication, drugs/alcohol, speed of car, tiredness
Question : A car is moving along an open road. Suddenly, a sheep walks into the road.
What do we call the distance the car travels before the driver puts their foot on the brakes?
Name one factor that could increase the distance the car travels in this time.
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
Medication, drugs/alcohol, speed of car, tiredness
Thinking distance = Stopping distance – braking distance
= 50m – 35m
= 15m

51. 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.

52. 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).
Stopping distance = thinking distance + braking distance
You could use a table like this for the road conditions task.
Graphs and bar charts are good ways of presenting information like this.
Speed
(mph)
Thinking time
(milliseconds)
Thinking distance
(metres)
Braking distance (metres)
Stopping distance 20 30
Road
conditions
Thinking time
(milliseconds)
Thinking distance
(metres)
Braking distance (metres)
Stopping distance
dry
wet
icy

53. 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

54. Investigating stopping distance Cars and carpet
light gate
toy car
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.
Measure the mass of the car
 Allow the car to run down the ramp and record:
(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.

55. 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
What effect did increasing the speed have on the stopping distance of the car?
What does this result lead you to think about real cars and car safety?
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 .

56. 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,

57. What can increase stopping distances?
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