The Science in Car Crashes

You may not be a fan of physics, but that does not mean that most of your everyday living does not obey the fundamental laws of physics. In a car crash, there are always two possible outcomes; the person that opens the door and walk away, and the other that gets carried away by emergency responders. Both of these situations all depend on how the laws of motion, based on physics, worked out that day.

Inertia, the tendency of a body to remain in motion until an external force acts on it, is one of the reasons seat belts are critical safety piece of equipment for anyone in a moving vehicle.

For instance, a body in a vehicle traveling at 100 miles per hour suddenly experience an external force, in the form of concrete road divider which forcefully decelerates the car to 0 mph, the body if unrestrained will continue to travel at 100 mph (in the same direction as the vehicle) until an object (steering wheel, vehicle's interior, windshield, etc.) stops the body from accelerating further.

Even if the person is not driving, it could still prove fatal or at least, you may get to endure what some doctors call a "partition face", which include broken noses, injured mid face or upper jaw, plus other injuries sustained as a result of slamming into the driver's back seat or any seat that you got thrown into. That is why it is advisable for everyone in a moving vehicle to secure themselves, even if you do not like living, you may turn out to be a missile/projectile that can harm others hers who may be more safety conscious in the event of a crash.

Crash and Inertia

Newton Second law expresses force, F, as a product of mass,m, and acceleration a.
F= ma

Acceleration, a, is the rate of change of velocity with time.

Therefore, F= m (∆v /t)

Multiplying both sides by t, we will have

Ft = m∆v

The product of mass and change in velocity (m∆v ) is the momentum.

Momentum defines the inertia in motion.

A 4000kg truck travelling at 25 miles per hour has the same momentum as a 1000kg bus cruising at 100mph. Even though the momentum of two moving vehicles may be the same, the severity of the crash between them will differ due to the impulse.

Impulse is defined mathematically as the product of force and time, i.e. Ft

Impulse, I = Ft

A car that was stopped suddenly experiences a more considerable stopping force (F) in a smaller time, t.
If we compare it with a car that stops over a longer time, t, that will have lesser force, F, acting on it as each momentum goes to zero.

To understand the effect of impulse in a collision, let us use an experiment. For this experiment, we need two eggs of the same mass, a wooden board, and some piece of cloth.

Hang the cloth on the two top ends, leaving the two bottom ends free, and make sure nothing but empty space is behind the fabric. Beside it, you can nail the board to the wall. Step back about 10m, one after the other using the same force, throw the eggs against the board and cloth respectively. You will find out that both eggs momentum will go to zero after hitting the two objects. But the egg you threw at the board will explode on impact and splash everywhere, the other thrown against the cloth may only sustain a crack or nothing at all.

What just happened? Did we not throw the two eggs with same force? We did. The reason is simple, the board stopped the egg with such massive force at such a short time, compared to the cloth that did the same thing with a lesser force over an extended period. The impulse on both eggs is the same. One experienced a more significant force over a short time while the other encountered a small force over much longer time.

The G-Force

The g-force is an equivalent pressure which applies to an object (or a human body) at sea level which is equal to the gravitational constant- 9.8 m/s²

1g is the average amount of force a person exerts either sitting, walking, or lying down, which prevents us from going into a free-fall.
For instance, a person that lay on his back feels the weight of his body, the same person in an aircraft or vehicle moving at 2G will feel twice the weight.

There is an interesting thing about G-force, it helps to express, technically, the amount of acceleration/deceleration body experiences rather than force, in that it does not require the mass of object involved to get this done. So an object of different masses has the same G-force acting on it.

As an example, a child and an adult, sitting in a vehicle with their seatbelt securely fastened. The car was travelling at say, 100km/hr when it suddenly slammed into a utility pole. The two bodies will experience different force exerted on their seat belt due to different masses, remember F=ma. The adult will have a more significant force acting on the seatbelt to secure his body from flying out of the car. The baby, on the other hand, will only experience little force on the belt due to the smaller mass.

But if we are to express the force in g-force, both bodies will experience same g-force acting on them irrespective of the different forces present due to their mass difference.

A similar phenomenon can be seen in a rollercoaster hurtling along its track. The rollercoaster exerts different forces on each rider according to their mass. But everyone on the rollercoaster experiences the same G-force. G-force conveniently allows for a mass-independent comparison for different situations while showing the scale of the force involved.

G-force and vehicle's crumple distance

A car that moves at 0-to-60 miles per hour (96.561km.hr) in 10 seconds exposes the driver to 0.27g.

1G= 9.8m/s²= 32.152231 ft/s²

Tesla Model S vehicles move from 0-60 mph in 2.39 seconds, pulling off 1.14g, which is slightly more than the average Earth gravitational constant.

Rollercoasters can go up to 6.3G. The human body runs a risk of going unconscious when subjected to G-force above 6G for more than a few seconds. This effect happens due to the blood flowing away from the brain.

Though, skilled professionals like the astronauts, fighter pilots, can withstand up to 9G and more, due to training-built tolerance inside the centrifuges. The other set of people with high G tolerance are professional NASCAR drivers.

To check the effect of a vehicle's front crumple distance in a collision, assume a car, X, was travelling at 30 mph and suddenly comes to a halt when it ran into a concrete wall. If the vehicle's front crumples one foot on impact with the driver secured to the seat with a seatbelt, the driver will experience a 30G force on that sudden deceleration. If in a second vehicle, Y, if the front end of the car is assumed to be less stiff than vehicle X, and it crumbles a distance of 2 feet, the deceleration G-force will now be 15G. That is a whopping minus 15G. Recall that impulse is Ft, the time the force acts on a body in the first instance (vehicle X) is halved in the second vehicle Y's impact.

Car manufacturers try to extend the time of impact using airbags, seatbelts, vehicle crumple zones, etc. to minimise the amount of time the force will act on the occupant's body/bodies in the event of a sudden deceleration in the case of a collision/crash.

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REFERENCES

• ^{Crumple zone}

• ^g-force

• ^{Impulse and Momentum}

• ^Impulse

• ^{Impulse Control - Studying Collisions}

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