Collisions and conservation laws
Collisions and conservation laws

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Collisions and conservation laws

6.2 Collisions on the roads

The growing number of cars on today’s roads makes it increasingly likely that each driver will be involved in at least one collision during their lifetime, making this a matter of personal interest for all of us (Video 1).

Download this video clip.Video player: Video 1 The physics of colliding cars.
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Transcript: Video 1 The physics of colliding cars.

ROBERT LLEWELLYN: All collisions obey the rules of nature. If police detectives know these laws, they can piece together what happened in a crash from the bits that are left. It’s the same whether it’s cars or atoms colliding. While the subatomic world may be strange and complex, scientists can always rely on some fundamental laws of collisions. Laws that apply from the mysterious atom to the world of the all too familiar.

ADRIAN HOBBS: More people are killed in road accidents than have been killed in all the wars and other types of accidents put together. We’re having in excess of one road accident per second worldwide. In this country, we have something like three and a half thousand people killed every year. It’s absolutely imperative that, in trying to understand what’s going on, that we do understand the physics. Just saying, well, I have a car, and it crushes the front, and not understanding why doesn’t tell me how I change things. So it is fundamental to all our work that we have to understand the laws of physics.

JON NEADES: Accident investigation is looking at the physical evidence that’s been left at the scene of a collision and establishing what has actually happened in the collision itself.

Okay, let’s have a look at the marks that we’ve got.

I’m Jon Neades, and I’m an ex-police officer. And now I teach accident investigation to police officers. You can see the black marks that have been produced as the tyre has run over the surface of the road. What that black mark is, certainly over this portion, a mark...

What is actually happening in the collision, what’s happened to the various objects, why we have marks on the road surface, why a vehicle behaves in a particular way. And it’s all based on the laws of physics.

ROBERT LLEWELLYN: In a skid, you may be out of control. But the laws of physics aren’t. Behind the chaos and confusion of a crash is the order and certainty of nature. If you understand these rules, you can work backwards and discover just how the metal got mangled.

It’s over a hundred years since the first person was killed in a road accident in Britain. This car may look lovely but it’s also deadly. Both car and driver would be written off in a crash.

ADRIAN HOBBS: I think that if we look at what’s happened in the whole development of the cars, we can say that by understanding how energy is absorbed, this has enabled us to move forward so that, in the unhappy event that you have an accident, then there’s a much greater chance that you’ll be protected in that accident. And without our understanding of physics, we would never have got to that point.

If we’re looking at energy, we have energy. We have kinetic energy if anything’s moving. Of course, when two cars crash into each other, they have a lot of kinetic energy. And we have to absorb that kinetic energy in the front structure of the car. If we don’t absorb it in the front structure of the car, it will have to be absorbed somewhere. And it’ll be absorbed by collapse of the passenger compartment. So we want to have the softest front structure that you can have but would absorb sufficient impact energy.

If you could design a car that had its front end, that would be like a spring, so that when cars collided, they crushed and then recovered. The problem there is the cars would then bounce back. And so the change in velocity on the cars would be much greater. There’s no point in stopping somebody and then saying, I’m now going to accelerate you backwards and increase your injuries. Now, you can’t do that perfectly. Cars will recover. But the ideal is cars which collapse and stay collapsed.

ROBERT LLEWELLYN: So car designers have a stark choice. In a crash, the energy either deforms the car or the people. Energy always has to go somewhere. It can’t just disappear. That’s what crumple zones are about. They allow the energy to go into bending metal. The front of the car crumples so the people don’t.

The total amount of energy in the Universe always remains the same. It’s conserved. Nature’s law on the conservation of energy is never broken. Scientists rely on it absolutely. But energy isn’t the only thing that obeys the law of conservation.

Motion is the key to understanding crashes. If my toy car were travelling at the same velocity as a real car, the real car would do a lot more damage. That’s because it’s got more momentum. Now momentum is the velocity of a vehicle multiplied by its mass. If two vehicles are travelling at the same velocity, the one with more mass has more momentum.

ADRIAN HOBBS: When we consider two cars hitting each other, then the first thing we have to consider is the momentum. So if you have one car that is twice the mass of the other car, then the momentum that that car has is its mass times its velocity. Both the cars are going at the same velocity. Then one car has twice as much momentum as the other car.

If you were to imagine something like a Mini hitting a truck - a very extreme example - you can see the very simple situation is if a truck is coming along at 30 miles an hour into a Mini at 30 miles an hour, the Mini will basically be going back at nearly 30 miles an hour. So it’s had a change in velocity of something like 60 miles an hour. And the truck will be going along still at almost 30 miles an hour. So it’s had a velocity change of virtually nothing.

JON NEADES: Well, momentum is one of those fundamental features that’s always conserved in every collision. The momentum is going to remain constant in a closed system. And we use that to establish, with two vehicles colliding, whatever momentum they had before the collision, exactly the same amount of momentum will come out of the collision at the other end. In fact, we use it in reverse. We know what happened after the accident, and we use that to predict what has happened and what the speeds of the vehicles are before the accident.

ROBERT LLEWELLYN: Just like energy, momentum is conserved. It stays the same. In a collision, momentum has to go somewhere. You can depend on that.

The police certainly do, as they attempt to work out the cause of the accident, and who’s to blame from the debris that’s left. Whether it’s on tarmac or green baize, behind every collision are the laws of conservation of momentum and energy. It’s these rules that let us predict how things behave during an impact.

End transcript: Video 1 The physics of colliding cars.
Video 1 The physics of colliding cars.
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