The Science in Car Crashes

in stemng •  7 years ago 

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


Pilots can undergo several Gs and not blackout [CC0] , from Creative Commons

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.

Car front showing crumple zone in a crash, By Janne. from Finland [CC BY-SA 2.0] , from Wikipedia Commons

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.


REFERENCES



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I am always asking my passengers to wear seatbelts.
When they say they are ok I calculate to them the inertial forces they will withstand from a car crash at 40km/h and calculate their weight in that moment. I explain to them that they will "fall" on my and my girlfriend's necks and then they will fly about a car length. They use their safety belts from that points onward.

I also had a very hard to convince friend and we agreed to hard brake check when he is not aware to see if he is able to take it.
I actually have better brakes than what the stock car has, in terms of performance and very good tires which I change constantly using real life performance reviews found at www.tyrereviews.co.uk , well before they reach the EU safety marks. I left foot braked him to a full stop on an empty road (Bosch ABS) out of the blue and he has worn the seatbelt ever since. In his car also.

I think the problem with not wearing the seatbelt or the erratic driving or tailgating is due to a lack of a visualization. Defensive driving lessons are hard to get, maybe impossible in developing countries.
How can we visualize and educate the drivers about these things, this is the question? How to convince the hard to convince, or the overconfident. "No, your BMW can't possibly brake in 0.2 seconds from the moment in which your mind realizes the car you tailgate emergency braked until you hit it with a difference of 50 km/h"

I am very wound up about it, you'll see it in my account description: "safe driver" :)

Even if the ABS can function in 0.2 seconds, can the driver's reaction be that fast? I had an accident that resulted in the car flipping off the highway to the adjacent bush. We got off unscathed due to the fastened seatbelt. Defensive driving? Anyone that can drive here can virtually be on the road as long as you have a driver's licence, a licence you can get without getting behind the wheel once in your life!

I was referring to that. The driver's response is way too slow for the speeds that we are travelling.
We either remove the driver from the ecuation or learn to cheat and give us more time, like keeping greater distance to the car in front and identifying potetial dangerous situations.

Driving while making some braking distance between our vehicle and the next really helps us stop and prevent a possible pileup.

@alexdory, that was an interesting piece.

Defensive driving is common sense just as giving the vehicle at the front some distance. These things don't even need to be taught imo.

If taught, it'd help. Same way people with survival training fare better doing crisis compared to those who don't have. It appears common sense is no longer common :)

Safe driver Alex!

This is hilarious going into the detailed calculations for your passengers to see the danger of non-compliance with seat belt usage! you do this as often as?
I also remember sometimes ago when I was in a car with my brother going to a destination of about half a kilometer away. I felt no need to use the beat because of the short distance.
Well, I wished I did. We had an accident before getting to the destination. I was in the hospital for some days as a result. Ever since then, I don't care if the journey is just 50m away, i will always use my seat belt.

I even brake check the kids the second they undo their seatbelts thinking that I can't hear the click :)
The seatbelt is the 50's invention, come on, it's high time we used it always..
I will write something, maybe tomorrow about the newer car security systems.

  ·  7 years ago (edited)

hahaha... I will watch out for your post. Remember to include some of your experiences with the use of seat belt

When I was very young I used to be paranoid about using seat belts, mainly because I watched movies where people get stuck in the car because of their seat belts after an accident. Later on I'll say I understood the science in car crashes as you'll put it and saw the importance of the seat belt.

It helps reduce the effect of impact on the body.

The airbags perform similar functions right?

That's pretty cool to know thanks for the tips by the way, have a nice..

sizuka ... we know you as Shizuka in our country ... you are famous here in the children.

kool I enjoyed applying physics and discovering the crashes. I believe we need to take help from biology to count the physical damage :D

That's right, the biology steps in when we analyse the damage to the body.

We are on the same page here ;)

@greenrun thank you for this enlightening info on physics

You are welcome.

Hello @greenrun

Using different real life scenarios to communicate the science behind car crashes, makes this article very easy to understand. Kudos.

Regards.

@eurogee of @euronation and @steemstem communities

Hello @eurogee, we can only try to make science as fun as we possible could.

Thats crucial for understanding. Thanks

Message gotten

Real life example to our boring lectures back in school.... Feels better now that it have a real life backing

Thanks @greenrun

Thanks for reading through it all.

@greenrun give me back my eggs first of all, I'll say the rest later...

These real life examples just made it look like the x, mph and the likes doesn't exist.
It was beautiful learning here.
Thanks

  ·  7 years ago (edited)

You can't make an omelette without breaking eggs, I just helped you break one. You can thank me later :)
Thanks for the compliment.

that's lot of science involved in that too interesting to know that too in details :)

  ·  7 years ago (edited)

This draws my attention to your previous post on the use of seat belts....
The experiment you describe here just stuck to my brain now, I am alking about the egg experiment, it really made the whole explanation much more easier and simple.

Need some clarification on the concept of lifters.
Uptil now , I still have some confusion as to the kind of force whether G or g that acts on us while in elevator . When the elevator is decelerating , it will look as if we have no weight but on acceration we feel heavy, still do not really understand this mechanism. I know it has some association with g and G but, honestly not too clear.

Sorry, I do not understand the question. If you could rephrase, that may help.

  ·  7 years ago (edited)

Aiit.
Normally when i step into an elevator I.e a lifter. I at first felt comfortable but immediately the lifter began to move up, I felt weightless and lost balance and if I could recall I nearly fell the first day I entered a lifter. I had to hold on to the iron support inside it so as to be stable.

I am now relating my experience to the concept of forces acting on us inside a lifter. The concept was taught by my physics teacher back then in high school but, I guess I was nonchalant about and didn't really pay much attention to it then. But now I do.
What I am kindly asking is that, At that point in time I felt weightless and lost my balance during the motion of the elevator, was gravitational force g still acting on me?
Just curious to know the reason for that weightlessness and lost of balance. It happens to almost anyone who steps into an elevator for the first time.

Thanks @greenrun for this post. At the beginning of this year; January 3rd to precise, I was involved in an auto crash, thanks to God and the seatbelts. Seatbelts are really important for both drivers and passengers.

Sorry about the accident, I hope you are better now. The importance of seatbelts is one that should never be downplayed in road safety objective.

She's totally fine though a bit shaky after the incidence. Could have been due to one of the downsides of ABS from what she relayed to me.

ABS usually give off warning in the event of a possible failure. It's there to help, though some may not pay close attention to those warning lights on the dashboard. Glad to hear she's ok.

This is great job.. I remember when I was in high school.. I was teach first law of motion..which you state above that a body is the state of rest unless external forces act upon it..
I really learn from your post because I don't know a body travel 100 mile in 1hour..

That is true, a body at rest will always be so until an external force acts on it.

My respect for seat belts increased the day I saw a video clip of different auto accidents. On impact, the car occupants 'flew' out. All of them.

I asked my brother, "Why are they flying out." I was truly ignorant.

"They didn't wear seat belts," my brother replied.

I became a believer.

Many lives will be saved if they understood the potential dangers in every moving car.

Many formulas I tried to forget in 2003 are in this post, but you brought back some good memories.

Thanks.

Why should you even try to forget the formulas? :)
Yeah, the seatbelt saves lives and that is true.

I was sixteen! Lol.
But we don't forget, do we?

Thank you for everything. You have made me aware of the importance of using seat belts while driving.

From my knowledge of the workings of seat belts.

When an accident occurs, the pretensioner mechanism will be active. The belt will be tightened to keep the body bounced. Seat belts and pretension systems can also be replaced, but should be careful as they are related to electronic systems in cars. Then after a few seconds, the power limiting mechanism works. The seat belt relaxes and exposes the body to a widespread airbag. This system works in harmony with the airbag. So the pretensioner mechanism and force-limiter mechanism work when the airbag expands.

The seatbelts do save lives.

Hi @greenrun!

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This reminds me so much of an episode in Big Bang Theory where Sheldon was trying to make his neighbor drive at a lower speed, when she refused, he started calculating how their mass and the speed of the car results unto momentum and how badly that means for them in case if a crash, sadly, the poor girl couldn't understand the geeky Sheldon. The beginning of this article would have been easier for her to get the point though.

I'm not very familiar with the G -Forces, but thanks to you, I'm an educated man now. Well done for this educating piece.

I don't understand Sheldon too :)
Thank you for reading.

Don't mind Sheldon, he's a special character.

That's why cars are equipped with airbags and seatbelts. Even modern day cars now have collision detection.
It's always better to be cautious.

Nice piece buddy

Collision detection is a great safety future, but I doubt it can help much when the vehicle loses control or when another car/truck slams into it. Thank you.

In other words, use the damn seatbelt if you don't want to end up like our egg on a wooden surface. Or we could just forget the whole thing and go at a constant 50km/hr. You'll be slow, but at least you won't die anytime soon.i

If none of this works for you, get outta the way, son.

This was a lot of physics Sir. Good read!

run...

If you go slow, what of the other driver doing fast and furious?

I don't think you are safe at that speed.

Yeah, read that. Given the rule of thumb that every other driver is mad. no one is entirely safe. I think i'll just stick to the seatbelt!

That rule looks good to me :)

I wish danfo drivers in Lagos could see this. They only use seatbelt to prevent LASTMA from catching them. By use i mean, they just wrap it across their body and sit on the other end.

I see drivers do that; they think it's the smart thing to do. It's only when an accident happens that the result shows how wrong they were.

Hehehe... Several times i have wrapped the seat belt around my body because their belts do not work. Brake application without belt may be dangerous.

And what will the Danfo drivers do when they see this? You think they'll read it... People that love money more than their lives. They are scared of paying money to LASTMA, but not scared of paying their lives for refusing to follow a simple safety rule.

Moral : Use your seatbelt the next time you want to wish... 🚶

It's indeed the science in car crash, I hope we all learn from it and keep safe. Kudos brother

Let's keep safe. Thank you.

The beauty of your articles is the analogical approach you employ. It makes what many could find abstract to become real.

Well done!

Nice one bro....thanks for enlightening us more about physics and also the important of seatbelt in a vehicle.

In some accident cases, when a child occupies the front seat, the air bag is said to hurt the child due to the force
how true is this ?

That was a very interesting read @greenrun. Very well explained, I enjoyed your examples.

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.

Whoa, I never would have imagined there are rollercoasters that can go up to 6.3G. Scary! I will think twice next time.

Though, skilled professionals like the astronauts, fighter pilots, can withstand up to 9G and more, due to training-built tolerance inside the centrifuges.

That is impressive to say the least.

Have a good day!

The human body runs a risk of going unconscious when subjected to G-force above 6G for more than a few seconds.

Though, skilled professionals like the astronauts, fighter pilots, can withstand up to 9G and more, due to training-built tolerance inside the centrifuges.

Astronauts can tolerate that blood flow thing on brain upto this level? Thats pretty impressive!

A goo post btw, green bro. :)

Wow! Thanks for the knowledge, it'll definitely come in handy.