1. Independent Study
We have done a lot of work on pressure, energy, work, power, forces, acceleration, velocity etc.
Can you now pull that all together and apply it in a single situation?
Read the following presentation (You can print out and annotate this power point rather than having to re-write it all. (in print settings choose 6 slides per sheet!)) Use p in your text book to help you too.
Now answer the questions in the section ‘Car safety’ in your booklet

2. To understand car safety
Be able to identify the features on a car that protect passengers in a crash
Be able to apply ideas of work and energy transfer to explain how the safety features work

3. F=ma a=(v-u)/t F=W/d
How are cars designed to reduce the force on a person during a collision?

4. Car safety features
Air bags, seat belts and crumple zones all act to prolong or increase the collision time (and distance). The occupants of the car are brought more slowly to a halt.
This means the change in velocity is spread over a longer period of time and so the deceleration is less.
From F=ma, this means the impact force on the occupants is also less. Also W=Fd -> Increasing the distance, decreases the force.

5. Seat belts
As well as increasing time and distance of collision, seat belts also help spread the force over a larger area, reducing the pressure.
Pressure = Force
Area
They hold the wearer
in place, stopping
them from being
thrown from their seat
Seat belts are designed to stretch slightly, to absorb some of the wearer’s kinetic energy

6. Crumple zones Crumple zones
Absorb some of the car’s kinetic energy when they deform, which would otherwise have been transferred to the passengers
Increase collision time and distance travelled during impact which reduces the forces acting on the passengers

7. Air bags
Increase the amount of time taken for the person to decelerate in a collision = Less force.
They also stop the passenger from hitting the dashboard or steering wheel during a collision
Air Bags

8. How does an airbag work? How air bags work:
Made of a flexible nylon bag
Have a sensor known as an accelerometer
In a crash (around -10g (g = 9.81)), a mass on a spring moves forward and makes an electrical contact
This triggers a chemical reaction (usually sodium nitride and potassium nitrate) producing nitrogen gas.

9. Key definitions - Car stopping distances
The total stopping distance of a car depends on 2 other distances:
Thinking distance – the distance the car travels between the driver seeing the hazard and applying the brakes
Braking distance – the distance the car travels between applying the brakes and the car stops

10. Identify the factors affecting stopping distance
Factors that increase thinking distance
Factors that increase braking distance

11. Identify the factors affecting stopping distance
Factors that increase thinking distance
Factors that increase braking distance
Higher speed
Tiredness
Poor road conditions
Alcohol and drugs
Poor condition of brakes
Distractions
Poor condition of tyres
Age of driver
Mass of car (more people or luggage in car)

12. Thinking distance For a vehicle moving at a constant speed:
Thinking distance (m) = speed(ms-1) x reaction time (s)

13. Braking distance
The relationship between speed and braking distance is not linear. When a car brakes, the kinetic energy of the car is transferred to thermal energy in the brakes. The work done by the friction force is equal to the transfer of energy:
Decrease in KE = work done = force applied by brakes x distance moved by car
½mv2 = Fd
Hence braking distance is proportional to the square of the car’s initial speed

14. You try
Questions on the sheet