Safety Features of Cars

Cars today are designed to rigorous safety standards to reduce the chance of a severe injury in the event of a collision
In your exam you need to know about the following safety features built into cars:
- Seat belts
- Crumple zones
- Airbags
Each feature is designed to increase the distance over which the energy of the passenger is transferred
- This reduces the impact force on the passengers and the severity of their injuries
The force applied to a passenger in a collision can be calculated using a rearranged form of the work done equation:

$force = \frac{work done}{distance}$

Where:
- Force is the force applied to the passenger by the impact of the collision (N)
- Work done is the work done by the safety device to reduce the movement of the passenger (J)
- Distance is the distance moved by the passenger as a result of the collision (m)

Car Safety Features

Car Safety Features, downloadable AS & A Level Physics revision notes

The seat belt, airbag and crumple zones help reduce the risk of injury to a passenger by increasing the distance over which the collision takes place

Seat belts
- These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop
- They are designed to stretch slightly to increase the distance the person moves in a collision
Airbags
- These are deployed at the front on the dashboard and steering wheel when a collision occurs
- They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact
Crumple zones
- These are designed into the exterior of vehicles
- They are at the front and back and are designed to crush or crumple in a controlled way in a collision
- This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions
- The crumple zones increase distance over which the collision takes place

Worked example

Upon impact with a wall, two passengers in a car have a kinetic energy of 3000 J. The driver's seatbelt stretches by 5 cm and the passengers by 2 cm.

Compare the force applied to the driver and passenger by their seatbelts as a result of the impact with the wall.

Answer:

Step 1: List the known quantities

Kinetic energy of both passengers, KE = 3000 J
Driver's seatbelt extension = 5 cm
Passenger's seatbelt extension = 2 cm

Step 2: Interpret the values given in the question

kinetic energy transferred = work done = 3000 J

Extension of driver's seatbelt in m = 5 ÷ 100 = 0.05 m

Extension of passenger seatbelt = 2 ÷ 100 = 0.02 m

Step 3: Recall the form of the equation to calculate the force applied in a collision

$force = \frac{work done}{distance}$

Step 4: Calculate the force applied to the driver

$force = \frac{3000}{0.05}$

$force = 60 000 N$

Step 5: Calculate the force applied to the passenger

$force = \frac{3000}{0.02}$

$force = 150 000 N$

Step 5: Compare the forces applied to the driver and passenger

$\frac{150 000}{60 000} = 2.5$

The force applied to the driver is 2.5 times smaller than the force applied to the passenger

Examiner Tip

You need to know how to use this form of the work done equation to analyse the safety features of cars.

Safety Features of Cars (WJEC GCSE Physics: Combined Science)

Revision Note