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Car Safety Features

The physics of how your car prevents you being killed in an accident

Ellen Wildig

on 7 November 2013

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Transcript of Car Safety Features

Car Safety
The physics involved in keeping you safe in your car
Like this.
An average road car is fitted with a number of safety features. The most common of these are;
Crumple Zones
Head Restraints
These are the most common features of an extensive list and it is these that will be explained in this presentation.
There are approximately 40,000 serious road accidents taking place in the UK each year. This caused around 25,000 serious injuries and 1850 fatalities in 2010. Worryingly between 90-95% of these accidents are caused by negligence on the part of the driver. 80% of the crashes involving drivers aged between 17-21 were their own fault.
A seatbelt is a standard safety feature fitted to every car sold in the UK. They are a legal requirement for all passengers in a car at any time it is travelling, everywhere in the UK. A seatbelt works by restraining the passenger in the event of an accident. The seatbelt locks into place when a large force is applied by a rapid change in momentum. This prevents the passenger from hitting the windscreen or the seat in front.
Physics of Seatbelts
When you crash the car comes to an abrupt stop, but you continue to move in the previous direction of travel. This is called inertia. The inertia of you and your car are completely seperate, which is why you continue to travel. The idea of a seatbelt is to counter your inertia by reducing your momentum before you hit the dashboard or windscreen, without causing you to undergo so much G-force that you are killed anyway. A seatbelt is positioned so that the force acts on less vulnerable parts of the body. As your inertia carries you forward your body exerts a sharp force on the seatbelt, which is designed to immediately lock into place. This reduces your momentum by applying an equal and opposite force over a period of time, sufficient to zero your momentum. As you can see this links directly to newtons second law. The change in momentum is caused by and is directly proportional to, the force applied by the seatbelt.
Suppose you weigh 70Kg and the car you are in is travelling at 20ms-1. Your momentum is equal to mass x velocity.
Therefore 70 x 20 =1400Ns
The car weighs 1100Kg, its momentum is,
1100 x 20 = 22000Ns
Now assume that the car is involved in a head on collision with a wall.
During the collision the car takes 0.1 seconds to stop.
Without a seatbelt the force exerted on you is calculated using the equation F = MV/ T.
Therefore 1400/0.1= 14000N
However with a seatbelt you may take fractionally longer to stop, say 0.25s
Now the force is equal to 1400/0.25= 5600N.
This is significantly less and is therefore a decrease the force exerted on you. However the major difference is in where the force is exerted. If your head hits the windscreen, it is likely to cause serious injury of death. However when wearing a seatbelt the same force is exerted over a much larger and much less vulnerable area.
Crumple zones
The crumple zone is an area at the front of any car which will fold in the event of a head on collision. Not only does this protect your legs by acting as a barrier between you and whatever you hit. It also lengthens the time of impact, lessening the force on you as a passenger by stopping you over a longer period of time. Again this shows how if the rate of change of momentum is decreased, by increasing the time, the force becomes proportionally less.
Assume your in the same 1100Kg car as before. You weigh the same and are travelling at the same velocity. The momentum of you and the car are therefore the same. Now assume that the car hits a tree.
Without a crumple zone the car will be stopped in around 0.05seconds. Over this time the force exerted on the car will be equal to momentum/time according to F= MV/ t
Therefore 1100x20= 22000Ns
22000/0.05= 440000N
However if a crumple zone is introduced the car will take longer to stop say 0.2 seconds. This means that the force is a quarter that which it was.
440000/4 = 110000N
An airbag is essentially just a huge cushion which inflates in the event of a crash. Although only inflated for a fraction of a second the airbag is designed prevent you from hitting any part of the car. An airbag is activated by a small explosion which takes place in the event of a large force being applied over a short space of time. They cause the driver or passenger to decelerate at a slower rate than hitting the dashboard making the overall stopping force lower. This lessens the likelihood of major injuries or death in a crash. An airbag takes approximately 40 milliseconds to deploy and remains inflated for around 2 seconds
In this presentation I will look at how your car stops you from being seriously injured or killed in a road accident. I will look specifically at 3 key safety features found in almost every road car in the world. These are
Crumple Zones
You will notice that all of these safety features try to achieve the same thing, to reduce the force on the driver during an accident.
The equation applied to the force in these accidents is; Force = momentum/time.
Unfortunately without the driver slowing down the momentum cannot be changed and therefore the safety features rely entirely upon lengthening the crash time for the occupants of the car to make the force less.
All of this is related to Newtons second law.
These are just a few of the many features that are included in modern cars to protect their occupants during a crash. Many others are used to produce a similar effect.
For example;
Head injury protection
Head Restraints
Others have been developed to allow the driver to maintain control of the vehicle in as many instances as possible.
For example;
Traction control
4-wheel drive
ABS (Anti-lock Braking System)
However accidents still cause approximately 2000 fatalities on UK roads alone. New ways to protect the passengers in a car are constantly being researched. However the driver of any vehicle has a responsibility to keep the car at a reasonable speed, drive in a way suitable to the conditions and generally to focus when behind the wheel.
Newtons Second Law
The rate of change of momentum of an object is proportional to the resultant force on it. In other words, the resultant force is proportional to the change of momentum per second.
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