P2.2 The kinetic energy of objects speeding up or slowing down

Car design & safety

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.

Key wordsForceEnergyWork donePowerPotential energyKinetic energyMomentumInertiaCrumple zoneSeat beltAir bag

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

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?

The authors point out that the introduction of: compulsory seat belts drink driving curbs child safety seats speed camerasas 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

Airbags

Front crumple zone

Collapsible steering wheel

Side impact bars

Front seat belts

Rear seat belts

Rear crumple zone

Car safety features

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.

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

Momentum Momentum (p) = mass (m) x velocity (v) p = mv

When a force acts on an object and moves it its momentum changes.

= v – u t

= m xIf the acceleration

is greater, what does it say about

the time?

If F is larger, whatcan you say aboutthe acceleration?a

a

F

More time means less force.

Newton’s second law

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)

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.

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.

New car with crumple zone.

Old car without.

Compare the impacts in slow motion…

Now consider the duration of the impact…

What is the effect of increasing the stopping time?

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

Stopping distance = +Thinking distance Braking distance

TirednessDrugsAlcohol

Bad road conditionsPoor weather conditions Poor condition of the vehicle eg brakes, tyres

Stopping distance

Braking systems

Anti-lock breaking 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.

Braking systemsRegenerative breaking

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 breaks 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.

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

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.