There is such a strange opinion that in a frontal impact, the speeds "add up". In the news about some accident, a police representative said that the speed of the cars was 100 km / h, which means a total of 200 km / h. Well, yes, in total: 100 + 100 = 200. You can't argue. And then what?
Interesting, of course, are not the numbers, but the real consequences of the strike. And you need to compare not just 100 and 200, but, for example, the consequences of a collision with a concrete wall. So, at head-on collision two identical cars at the same speed of 100 km/h, each effect for either of these two cars will be, as many believe, the same as hitting a concrete wall at 200 km/h. And this is a very dangerous delusion, in my opinion. The effect will be the same if you drive into a concrete wall at 100 km/h. Exactly 100, not 200!
In general, the thoughtless addition of numbers resembles the cartoon "Squad America: World Police". In it, about some terrible terrorist attacks, they said something like: “It will be 10 times worse than 9/11.” Then someone said: “9110 is some kind of horror!!”. I can't vouch for accuracy, but the meaning has not changed. 911 what? 9110 what? So here - 200 km / h of what? Relative to the Sun, we generally move at a speed of 30 km / s, and nothing. Moreover, if you accelerate to 200 km / h and then slow down smoothly, what will happen is not the same as sharply vtemyashitsya into a concrete block. Those. It's not the speed that's important, it's the timing of that speed. The maximum acceleration experienced by occupants of the vehicle during braking, impact, etc.
Probably, thoughts about the addition of velocities come to mind in connection with residual memories from physics. But there, no one adds speed thoughtlessly. There is conservation of energy, there is conservation of momentum. There are accelerators on colliding beams. But we are not interested in the behavior of systems of bodies, but in the “sensations” of one body. The sensation of the body will just be the maximum acceleration, and not the total energy-mass-momentum.
In the case of a collision with a concrete block and in the case of a collision with an oncoming car, from a practical point of view, it can be assumed that the speed extinction time will be the same. And the acceleration will be the same. This means that it makes no difference what to drive into - a concrete block or the same car traveling to a meeting at the same speed. There are no additions of velocities here and cannot be. This is a delusion, and a very dangerous one, it is now easy to see.
Of course, you need to understand that a sliding blow is better than a direct frontal one. That instead of an oncoming strike, it is better to prefer a hit on a passing car - it is softer. That a hit on a passing car is softer than a hit on a “passing” concrete block. In general, it is important to understand what dangers lurk on the road, and to see which of them are more terrible and which are less. To save your life, your health will have to make a choice. Knowledge is required to make an informed choice. But they don't give us. But what can I say: even traffic police officers, people who are directly related to traffic safety, don’t even have them.
It is generally accepted that frontal collision speed cars are summed up and the result will be the same in a collision with a concrete wall at the same total speed. But is it? The MythBusters decided to conduct an experiment to establish the truth while conducting three crash tests and smashing four Daewoo Nubira cars.
« ...Remember how we pushed two cars facing each other when the speed of each of them was 80 km/h. And you said it's the same thing if one of them crashed into a wall at 160 km/h. The fans were indignant, indignant, they said that you were mistaken.
They argued that the collision of two cars at a speed of 80 km / h is not equivalent to one of them crashing into a wall at a speed of 160 km / h. And it is equivalent to if one of them drove into the wall at a speed of 80 km / h. So what do you say?
- I think we should check.
- Let's check.
So, the argument develops around Newton's third law: for every action there is an equal and opposite reaction.
- And what do the fans want? They want us to use two full size cars. But I think that we should shed light on the laws of physics with a full scale experiment.
- In more controlled conditions.
- Exactly!
- And then we will break these cars».
(Leaving out the details, let's say that the result of the test in the laboratory indicates that the Fans were probably right).
Video #1 in Russian from MythBusters ("MythBusters")
Does speed add up in a frontal collision?
https://www.youtube.com/v/RowK7Ytv9Ok
But this, of course, was not enough. It's time to smash the real machines by confirming the test results in the field. The location of the event is Arizona.
For the test, they chose the Daewoo Nubira, which is going to be smashed against a wall at a speed of 80 km / h.
1280 feet is the length of the Nubira's path to the wall. Of course, the car will be without a driver and it will be accelerated with the help of electricians - this is what rails are for. On back seat and installed in the trunk special device, which captures all the data. In general, something like a black box in airplanes.
So, the length of the whole "Nubira" is 15 feet.
https://www.youtube.com/v/dMVeq6P5s9E
Video No. 2 on the topic: "Do speeds add up in a head-on collision?"
After the impact, the length of the car was reduced to 11 feet. And I'll tell you right away that if we crash this car at a speed of 100 miles per hour into a wall, the damage will be much more significant.
So now the same wall, the same car (only yellow color) - and a speed of 160 km / h.
Let's see how strong the compression will be at a speed of 160 km / h. We just lost the power of speech: "Nubira" has become two times smaller. Was 15 feet - became 8!
So, we believe that if you double the speed, then the damage is doubled. But physics tells us something else: if the speed is doubled, the damage is approximately quadrupled!!!
Our sensors recorded that the reaction force coefficient in the second case (100 mph) increased by more than three times compared to the first (80 km/h).
In a word, physics operates during the collision, but one does not have to be a scientist to understand the consequences. Machines, or rather their condition, speak for themselves.
But, it's time to move on to the main event: if the cars are pushed in a frontal attack, at a speed of each of them 80 km / h, what will they look like?
It's no secret that there are many myths associated with car safety. Forums, LiveJournal and offline discussions are full of advice on which car is safer and how best to behave in emergency. Most of these tips, if not useless, then meaningless - a person advises buying a "five-star" car according to EuroNCAP, but why, how, in fact, and what these stars mean - cannot be explained. In particular, almost no one understands how "stars" correlate with the probability of being seriously injured in a particular type of crash at a particular speed. It is clear that the more stars - the better, but how much is "better" and where is the safe limit? LiveJournal User 0serg countedhow, on what and where it is safer to crash , and smashed to smithereens the theory of EuroNCAP-ovskih "stars".
One of the most widespread myths is that very often, when talking about a frontal impact of cars, the speeds of these cars add up. Vasya was driving 60 km/h, and Petya flew out of the oncoming lane at a speed of 100 km/h; This is the biggest mistake. Real" effective speed shock" for machines will usually be approximately arithmetic mean the speeds of Vasya and Petya - i.e. near 80 km/h. And it is this speed (and not the philistine 160) that leads to wrecked cars and human casualties.
"On the fingers" what is happening can be explained in this way: yes, upon impact, the energy of two cars is summed up - but two cars also absorb it, so each car accounts for only half of the total impact energy. The correct calculation of what happens upon impact is available even to a schoolboy, although it requires a certain ingenuity and imagination. Imagine that cars at the moment of impact slide along a flat highway without resistance (considering that the impact occurs in a very short time and the impact forces acting on the cars are much higher than the friction forces from the side of the asphalt - even with intensive braking, this assumption can be considered quite fair). In this case, the movement upon impact will be completely described by a single force - the resistance force of crushed metal bodies. This force, according to Newton's 3rd law, is the same for both machines, but is directed in opposite directions.
Let us mentally place a thin, weightless sheet of paper between the machines. Both resistance forces (the first machine and the second) will act "through" this sheet, but since these forces are equal and opposite, they completely cancel each other out. And therefore, throughout the impact, our sheet will move with zero acceleration - or, in other words, with constant speed. In the inertial coordinate system associated with this sheet, both machines seem to "crash" from different sides into this motionless sheet of paper - until they stop or (simultaneously) fly away from it. Do you remember the EuroNCAP technique where cars crash into a fixed barrier? Hitting our hypothetical "sheet of paper" in our special system coordinates will be tantamount to hitting a massive concrete block at the same speed.
How to calculate the speed of a sheet of paper? It's quite simple - just remember the mechanics of collisions from the school curriculum. At some point, both cars "stop" relative to the coordinate system of a sheet of paper (this happens at the moment when the cars begin to fly apart in different sides), which allows us to write down the law of conservation of momentum. Considering the mass of one car m1 and speed v1, and the other - m2 and speed v2, we obtain the speed of a sheet of paper v by the formula
(m1+m2)*v = m1*v1 - m2*v2
v = m1/(m1+m2)*v1 - m2/(m1+m2)*v2
For a collision in the "following" direction, the speed of the second car should be considered with a "minus" sign.
Relative speeds machines relative to paper (i.e. "equivalent speed of impact on a concrete block") are respectively equal to
u1 = (v1-v) = m2/(m1+m2) * (v1+v2)
u2 = (v+v2) = m1/(m1+m2) * (v1+v2)
So the "equivalent speed" frontal impact is indeed proportional to the sum of the speeds of the cars - however, it is taken with a certain "correction factor" that takes into account the ratio of the masses of the cars. For cars of equal mass, it is equal to 0.5, i.e. the total speed must be divided in half - which gives us the “arithmetic mean” mentioned at the beginning of the note, typical for such accidents. In the event of a car collision different weight the picture will be significantly different - a "heavy" car will suffer less than a "light" one, and if the differences in mass are large enough, the difference will be colossal. This is a typical situation for accidents of the "passenger car crashed into a loaded truck" class - the consequences of such an impact for a passenger car are close to the consequences of an impact at full "total" speed, while the "truck" gets off minor damage, because for him, the "equivalent impact velocity" turns out to be equal to a tenth or even a twentieth of the total velocity. 
So, we have learned to calculate the "equivalent impact speed" using a very simple formula: you need to add the speeds (for an impact in passing direction- subtract), and then determine what proportion of the mass is the ALANGER's car from the total mass of your cars and multiply this coefficient by the calculated speed. Estimated coefficient values:
Cars of approximately the same weight category: 0.5
Small car vs passenger car: small car 0.6, passenger car 0.4
Subcompact vs Jeep: Subcompact 0.75, Jeep 0.25
Car vs jeep: car 0.65, jeep 0.35
Car vs truck: car >0.9, truck<0.1
Jeep vs truck: jeep >0.8, truck<0.2
For example, a Porsche Cayenne jeep weighing 2.5 tons at a crossroads crashes at a speed of 100 km/h into a 1.3-ton Ford Focus II that has barely begun a left turn. The total speed is 100 km/h, the equivalent impact speed for the Cayenne is 35 km/h, and for the FF it is 65 km/h.
The main threat to the life of the driver upon impact is determined (if he is fastened) by the deformation of the car interior. This deformation, in turn, is approximately proportional to the absorbed impact energy. And this energy is determined by the good old formula "em ve squared in half", i.e. already for 80 km/h it will be 1.5 times more than the "nominal" EuroNCAP energy, at 100 km/h - 2.5 times more, at 120 km/h - 3.5 times more, at 140 km/h h - almost 5 times more.
That's why RThe real safety of the EuroNCAP "stars" is ensured only with an effective impact speed of less than 80 km/h!
In other words, everything above 80 km / h is potentially life-threatening, regardless of vehicle type. "Unfortunate racers" in expensive cars are really saved only by the "reducing factors" mentioned above - even at a total speed of 200 km / h, they have been shown to usually reduce the effective speed of a significantly heavier car to 80 km / h or less. Yes, and the brakes usually allow you to have time to drop at least 20-30 km / h (and more often - more) at the last moment - hence the apparent safety of expensive jeeps. But when you hit a solid immovable obstacle or a truck, everything will end much sadder.. The strength of the car at 100 km / h is a very conditional concept! Speeds up to 80 km / h on modern cars are almost safe in any situation, but a driver flying at a speed of 140+ km / h is most likely a killer or suicide.
It should be noted that this feature is associated with a characteristic myth about the "low safety" of passenger cars, especially small-capacity and Russian-made ones. Usually, eloquent examples of a head-on collision of such a car with some executive car or jeep are cited to confirm it - but I suppose you can already guess that the main reason for such a nightmare is not so much the "low strength" of these cars as low weight, due to after which the consequences for a light car will obviously be many times stronger than the consequences for a heavy one. The quality of the implementation of the passive safety of the machine in such strikes is already fading into the background. However, in all other accidents (departure from the highway, hitting a truck, hitting about the same car), the situation will not be so dramatic. For heavy cars, the exact opposite is true.
Briefly - about unfastened seat belts. When hitting an obstacle, an unbelted person flies onto the steering wheel at a speed approximately equal to the effective impact speed. The speed gained by a person falling from the fifth floor of a building when hitting the ground is less than 60 km/h. About half survive. The speed gained by a person falling from the ninth floor is about 80 km/h. Units survive. Airbags and a well-chosen posture help to mitigate the consequences (making survival at 60 km / h very likely, and at 80 more likely), but I would not count on them much. Literally plus 40 km / h to a relatively safe value (which, as I already mentioned, is closer to 60 in typical accidents) - and you are a guaranteed corpse, no matter what you do, and no matter how advanced the security system in the car is. The margin of safety for those fastened is much higher - plus 100 km / h to a safe speed will be critical there, and it will not be so easy to go beyond these limits. In unfortunate situations (departure to the side of the road or under a truck), both numbers should be divided in half.
Practical Tips:
1. Do not exceed the speed limit. The chances of dying after 120 km / h increase VERY quickly, although for heavy vehicles the safe upper limit is usually slightly higher - alas, at the expense of the safety of others.
2. If you exceed - buckle up. Although for relatively low speeds (0-100) without a belt there are quite a lot of chances to survive, in the speed range of 100-140 in an accident, often unfastened = corpses.
3. A modern heavy car is almost always much safer. in accidents with lighter vehicles. This consideration does not apply to accidents involving trucks or running off the road. Just do not forget that a large mass does not always compensate for poor passive safety - junk 20 years ago is so much worse than modern 4-5 "star" cars that there is little that can save it in an accident.
4. A hit on a fixed heavy obstacle on the side of the road is more dangerous for a heavy car than a head-on collision. For a light car, the opposite is true.
5. Impact on a stationary car, and even more so - a car moving in the same direction always much safer than hitting a fixed heavy obstacle on the side of the road.
6. If you see that there will be an accident now, and it’s too late to dodge, slow down, as prescribed by the traffic rules. Trying to pull over to the side of the road without slowing down is usually at least as dangerous.
7. The only exception to paragraph 6 is the case when a truck flies in your forehead at high speed - it’s better to do anything here, but get out of its way. But I have never encountered this situation in real life (and in order not to fly out onto trucks at high speed - see point 1).
Among motorists there are a lot of plausible myths that a large number of people believe in. We have already written about many myths on the pages of our publication. Today we want to talk about the most common myth - about the addition of the speeds of two cars in a frontal impact. Let's dispel this myth once and for all.
Somehow it so happened that many people believe that if two cars collide head-on, then the impact energy will correspond. That is, as many motorists believe, in order to understand how strong a frontal impact will be, you need to add up the speeds of both cars involved in an accident.
To understand that this is a myth, and to calculate the force of a frontal impact and the consequences for cars involved in such an accident, we need to make the following comparison.
So, let's compare the consequences for cars in different accidents. For example, each car is moving towards each other at a speed of 100 km/h, and then they collide head-on. Do you think the consequences of a frontal impact will be more serious than from at the same speed? Based on a common myth that has been circulating for several decades among people who only half know physics (or are not familiar with it at all), then at first glance, the consequences of a frontal impact of two cars at a speed of 100 km / h will be more deplorable than a car at the same speed against a brick wall, since the frontal impact force will supposedly be greater due to the fact that the speeds of the cars in this case need to be added. But it's not.
In fact, the force of a frontal impact of two cars at a speed of 100 km / h will correspond to the same force as when they hit a brick wall at a speed of 100 km / h. This can be explained in two ways. One is simple, which even a schoolboy will understand. The second is more complex, which not everyone will understand.
SIMPLE ANSWER
Indeed, the total energy that must be dissipated by crushing the metal of the body is twice as high when two cars collide head-on than when one car hits a brick wall. But in a head-on collision, the distance of crushing the metal of the bodies of both cars increases.
Since the bend in the metal is where all this energy is going to be absorbed twice as much as it will be absorbed by two cars, as opposed to hitting a brick wall where the kinetic energy will be absorbed by one car.
Thus, the deceleration rate and force of a frontal impact at a speed of 100 km/h will be approximately the same as when hitting a brick immovable wall at 100 km/h. Therefore, the consequences for two cars moving at the same speed and colliding head-on will be about the same as if one car crashed into a stationary wall at the same speed.
MORE DIFFICULT ANSWER
Let's assume that the cars have the same mass, the same deformation characteristics and perfectly at right angles collide head-on and do not fly far from each other. Let's say both cars stop at the collision point. Thus, moving, for example, at a speed of 100 km/h, each car will stop on impact from 100 to 0 km/h. In this case, each car will behave in exactly the same way as if each of them collided with a stationary wall at a speed of 100 km / h. As a result, both cars will receive the same damage in a perfect frontal impact as if they hit a wall.
To understand why exactly the same damage, you need to conduct a thought experiment. To do this, imagine that two cars are traveling at a speed of 100 km/h towards each other. But on the road between them there is a thick, very strong, immovable wall. Now imagine that both cars simultaneously crash into this imaginary wall from opposite sides. Each at this moment simultaneously stops from 100 km / h to 0 km / h. Since the wall on the road is very strong, it does not transfer the impact energy from one car to another. As a result, it turns out that both cars hit a standing wall separately, without affecting each other.
Now repeat this thought experiment with a thinner and not very strong wall, but able to withstand the blow. In this case, if the blow is from two sides at the same time, the wall will remain in place. Now imagine instead of a wall a sheet of a durable piece of rubber. Since two cars hit it at the same time, the rubber sheet will stay in place since both cars will hold the rubber in place at the same time they hit it. But a thin sheet of rubber cannot slow down any car, so even if you remove a sheet of rubber between cars that collide head-on, each car still stops at the moment of impact from 100 km / h to 0 km / h, that is just as if one car crashed into a solid, immovable wall at a speed of 100 km / h.
Is the impact energy and consequences the same in a collision with a stationary car or a stationary wall?
This is another common myth among car enthusiasts, which is related to the fact that if at a speed of, for example, 100 km / h, collide with a standing car, then the impact force will be exactly the same as if the car flew into the air at a speed of 100 km / h into a fixed wall. But this is not so either. This is pure water myth, which is based on ignorance of elementary physics.
So, imagine the situation that one car is moving at a speed of 100 km / h and at full speed collides with exactly the same car standing on the road. At the moment of impact, one car, continuing its movement, will push the other car. As a result, both cars will fly away from the collision site. At the moment of impact, the kinetic energy will be absorbed by the deformation of the body of both cars. That is, the impact energy will also be shared between the two cars. In the case of a blow to a fixed wall of one car at a speed of 100 km / h, only one car will have deformation of the body. Accordingly, the impact force and its consequences for the car will be greater than when hit at the speed of one car into another, which is standing still.
