The study of physics has had a large impact on the development of road safety. The study of Newtons 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 industrys 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 cars 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. (Newtons 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.
Inertia reel seatbelts
Inertia reel seatbelts have a cylinder that stores extra belt material on a spool. The spool turns freely normally for movement. In the event of a sudden change in velocity, a pendulum in the reel plunges forward to lock and restrain the occupant. It is named inertia reel due to the inertia of the pendulum to cause belt locking.
Different types of seatbelt
The lap seatbelt is tightened over the pelvis and absorbs force over a sizeable area. However they do not prevent the head and upper body from lurching forward in response to deceleration and are inadequate especially for front passengers, who risk hitting the windscreen.
The lap sash seatbelt combines a lap seatbelt over the pelvis as well as over one shoulder across the chest. This greatly limits movement of the upper body and head as well as spreading the stopping force over a larger area.
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 drivers 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.
Padded headrests provide protection in rear end collisions. When a car is rear-ended, a large net force pushes the car and it accelerates forward. The inertia, based on Newtons first law, pushes the passenger into the seat. Without a head rest for support, the head remains at rest until the spine pulls it forward. This sudden, sharp acceleration for the head causes it to fling back and results in hyperextension and whiplash injuries.
Many cars are designed to crumple at the front and rear. Crumple zones increase the time interval during which the momentum of the car changes during a collision, thus lessening the forces. Newtons second law (f=ma) can be applied, as increased ?time will result in a decreasing rate of deceleration a and consequently reduce force.
Crumple zones are built using the integration of steel and fiberglass in the front and rear end assemblies of the automobile. Rigid structures between crumple zones protect the passenger compartment.
In a collision, the crumple zones deform to slow the actual impact. The car doesnt regain all its original kinetic energy, as some of this is converted to heat and sound energy to reduce damage to passenger area through smaller forces. As crumple zones are placed in strategic locations, the collapse is controlled and energy from impact is directed away from passenger area.
Speed humps and low speed zones
Local councils introduced Speed Humps to reduce the overall speed of cars. Evidence shows that slower moving cars cause less damage to occupants if a crash results. It has been estimated that risk of death or serious brain damage doubles with every 16km/h over 80km/h. Speed humps restrict drivers from speeding as hitting the hump too fast can cause car damage.
Low speed zones also reduce the speed of cars. A reduction of speed will reduce the chance of an accident as drivers have more time to react as well as decreasing the change in momentum and associated inertial forces in collisions. The police enforces speed signs and limit zones and speeding is monitored by speed cameras. In NSW, the general urban limit is 60 km/h for roads in built up areas where there are pedestrians.
Crash barriers are road design features that absorb the impact of collisions. They are constructed out of steel, concrete or wire. Some types of rigid crash barriers cannot absorb much of the kinetic energy of the vehicle, but keep the vehicle on the road and prevent crashing into more dangerous roadside hazards or cars from opposite direction. Crash barriers should be relatively weak with energy absorbing structures so that they can deform easily and transfer large amounts of kinetic energy to them in collisions. For barriers shielding against hazards such as trees, they need to be a fair distance away, as space is needed for it to deform.
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.