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The study of physics has had a large impact on the development of road safety. The study of Newton's Laws, impulse and momentum help to reduce initial momentum or decrease impulse (rate of change of momentum) to protect passengers.

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Year11 Physics Research Assignment

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The study of physics has had a large impact on the development of road safety.  The study of Newton’s Laws, impulse and momentum help to reduce initial momentum or decrease impulse (rate of change of momentum) to protect passengers.  Technological advances and studies of vehicle crashes increase the automobile industry’s understanding of the forces involved and ways of reducing effects of collisions.  Within vehicles, many car safety devices are installed, such as seat belts, air bags, crumple zones and head rests.

Modern road design reduces the initial or rate of change of momentum and consequently the impact of collision through crash barriers, speed zones and speed humps.

Car safety devices


Seatbelts are designed so that passengers are restrained instead of continuing to move forward at the car’s speed when the brakes of a moving vehicle are applied.  In this situation, there is a tendency for a body to resist changes to its motion, called inertia. (Newton’s first law)  The stopping force is applied to the more durable parts of the body such as hips, chest and shoulder.  This is instead of the human head crashing into windshield if no seat belt is worn. image00.png

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Shoulder harnesses restrain upper torso movement even further. Two are belts fall over the shoulders and an optional strap lies over the sternum.  It is effective in the way force is spread over both shoulders compared to one shoulder in lap sash.  Shoulder harnesses are usually used in child restraint systems and racing cars.

The nylon webbing material in seatbelts is slightly flexible so that the stop is not as abrupt.


Airbags provide an extra degree of protection in a collision by cushioning passengers in a collision, greatly limiting fatalities and serious injury.  They are designed to increase the time interval during which the driver’s momentum decreases in a collision to decrease the net force of the driver.  They inflate when crash sensors detect large deceleration. Sensors then ignite the sodium azide, producing sodium compounds and nitrogen gas for a reactive explosion.  The airbags inflate rapidly to cushion the impact of the passenger against the steering wheel, dashboard or windshield.  Airbags are to be used with seatbelts, not as a replacement.  A disadvantage of airbags is they provide no protection against side-on hits, only frontal collisions.  More expensive cars are developing side airbags to combat this.image05.jpg


Head rests

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When a vehicle collides, the barriers deform and stop the vehicle through a plowing action, keeping the reaction forces relatively low.  The impulse (change in momentum) is reduced, as the vehicle takes longer to slow down.  

The disadvantage of crash barriers is they may cause vehicle damage (crumpling) and rigid ones may deflect a car into the opposite lane.  

The advance of road safety

The development of vehicles with greater size, power and momentum has resulted in the need for improvements in car safety devices and modern road design.  The study of physics has allowed safety features in both the interior and exterior of cars to prevent large forces acting on passengers in collisions.  This is why researchers and engineers are continually turning to laws of physics for advice.  Road design features reduce initial velocity to reduce the risk of accidents as well as its impact on the passenger.  Automotive safety companies are continuing to develop devices to improve vehicle safety, including inflatable seatbelts, 4-point seatbelts and knee airbags to name a few.  With the large percentage of the population driving - forces, momentum, impulse and energy are all factors that must be controlled as our lives depend on it.

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