The document discusses various car safety features and the physics behind how they work to reduce forces and injuries during collisions. It explains that seatbelts, airbags, crumple zones, and other features work to increase the time over which a person decelerates during a crash, which reduces the force on the body. Regenerative braking is also discussed as a way to capture some of the kinetic energy lost during braking.

1. P2.2 The kinetic energy of
objects speeding up or slowing
down
Car design &
safety

2. You should be able to:
■ evaluate the benefits of air bags, crumple
zones, seat belts and side impact bars in
cars.
■ evaluate the benefits of different types of
braking system, such as regenerative
braking.

3. Key words
Force
Energy
Work done
Power
Potential energy
Kinetic energy
Momentum
Inertia
Crumple zone
Seat belt
Air bag

4. Why do you think
most car deaths
occur with people
aged 20-29?

5. Around 1.3 million people die on the world's
roads every year, and on the basis of current
trends, deaths caused by vehicle collisions are
expected to become the fifth leading cause of
death by 2030.
The annual number of car crash deaths in
England and Wales has plunged by 41% over the
past 50 years, despite the increase in drivers on
the road, reveals research published in 2013 in
the Emergency Medicine Journal.
Why the decrease?

6. The authors point out that the introduction of:
compulsory seat belts
drink driving curbs
child safety seats
speed cameras
as well as the development of specialist trauma centres,
will all have helped to drive down road traffic fatalities.
YET …
WHY?
Cars have become safer, but safety comes at a cost –
more expensive cars have more safety features

7. Airbags
Front crumple
zone
Collapsible
steering wheel
Side impact
bars
Front seat belts
Rear seat belts
Rear crumple
zone
Car safety features

8. Inertia: the resistance an object has to
a change in its state of motion.
If the car comes to a sudden stop – the
passengers will keep moving until another force
stops them.
If it’s the windscreen, steering
wheel or dashboard it can
cause a lot of damage.
The seat belt helps to keep
them in their seat
and an air bag will cushion the
blow.

9. Momentum
All moving objects have momentum. The greater the
mass and the greater the velocity then the greater
the momentum.
Momentum (p) = mass (m) x velocity (v)
p = mv
In a collision the momentum is the same before
as after the collision (provided there are no
external forces)
Momentum before = momentum after

10. Momentum
Momentum (p) = mass (m) x velocity (v)
p = mv
When a force acts on an object and moves it its
momentum changes.

11. = v – u
t
= m x
If the acceleration
is greater, what does
it say about
the time?
If F is larger, what
can you say about
the acceleration?a
a
F
More time means less force.
Newton’s second law

12. Seat belt
The seat belt exerts a force to keep the driver and
passenger in their seats and prevent them going
through the windscreen.
The seat belt stretches slightly which increases
the impact time (t) and so reduces the force (F)

13. Air bag
With an air bag the time
for a person to
decelerate is greater
than if they hit the
windscreen.
The impact time (t), the
time to come to a halt,
is increased so the force
(F) is decreased.
The air bag spreads the forces on the body
across a larger area.

14. Crumple Zone
• The crumple zone is
designed to absorb
energy from a collision
and reduce the force of a
collision.
• Folding during a collision
crash increases the
impact time.
• Time (t) to come to a halt
is increased so the force
(F) is decreased.

15. New car with crumple zone.
Old car without.

16. Compare the impacts in slow motion…

17. Now consider the duration of the impact…
What is the effect of increasing the stopping time?

18. The stopping distance of a vehicle is the sum of the distance the
vehicle travels during the driver’s reaction time (thinking distance) and
the distance it travels under the braking force (braking distance).
Stopping distance = Thinking distance + Braking distance
Stopping distance

19. Stopping distance = +Thinking distance Braking distance
Tiredness
Drugs
Alcohol
Bad road conditions
Poor weather conditions
Poor condition of the
vehicle
eg brakes, tyres
Stopping distance

20. Braking systems
Anti-lock braking system (ABS) – helps to stop
skidding and so helps keep the car under control.
Brakes work by converting the car’s kinetic energy
into heat energy.
When the brakes of a vehicle are applied, work
done by the friction force between the brakes and
the wheel reduces the kinetic energy of the vehicle
and the temperature of the brakes increases.

21. Braking systems
Regenerative braking
By using the car's energy to drive a generator to charge a battery
saves the energy that can be used later.
The energy from the regenerative braking is energy that would
have gone to heating up the brakes in non-hybrid cars.
By doing this the hybrid gets more useful work out of the original
input energy (either gas or electric).
Hybrid cars have an electric engine and a
petrol engine. This type of car is often fitted
with a regenerative braking system.
A regenerative braking system not only slows
a car down but at the same time causes a
generator to charge the car’s battery.

22. A car travels a distance of 1000m in 40s (constant speed).
What is the speed of the car? s = d/t = 1000/40 = 25 m/s
What is the acceleration if the car stops in a further 5s?
a = (v-u)/t = (0 - 25)/5 = - 5 m/s2
If the car has a mass of 1000kg, how much kinetic energy did it have during the
40s?
KE = ½ mv2 = ½ x 1000 x 252 = 312500 J
= 312.5 kJ
How much work is done by the brakes in stopping the car?
Work Done = Energy Transferred = 312.5 kJ
What is the average retarding force acting on the car as it stops?
Force = mass x acceleration = 1000 x 5
= 5000 N

28. Extended question
In 1983 it became a legal requirement to wear a seatbelt whilst
travelling in a car in the UK. Since then new car designs have
developed to increase the safety of the driver and passengers.
Outline these safety designs.
You should include:
• A description of the safety features of a vehicle
• An explanation of how they protect the people in the car
• The physics concepts when the vehicle stops suddenly or crashes
• Any ideas you have on future vehicle safety features
• Diagrams to support your description.