Car Safety and Collisions

A modern car is one of the safest ways to travel, and yet the physics of a crash is brutal: in a collision, a person moving at motorway speed must be brought to rest in a fraction of a second. What decides whether they walk away or are seriously hurt is not really whether they stop, but how the stopping happens — how large a force acts on their body and for how long. This lesson, part of the "Physics on the move" section of Topic P6 (Global challenges) of OCR Gateway Combined Science A, looks at the forces involved in a collision and explains how the safety features built into every car — crumple zones, seat belts and airbags — reduce the force on the occupants by making them stop over a longer time.

By the end of this lesson you should be able to describe what happens to the forces on the occupants during a car crash, explain the general idea that a bigger force is felt when a change in motion happens over a shorter time, and explain how crumple zones, seat belts and airbags reduce injury by increasing the time taken to stop. (You met the equation for momentum, $p = mv$p=mv, in Topic P2; here the treatment is qualitative — you do not need to calculate momentum.)

This lesson is largely AO1 (recalling car-safety features) and AO2 (applying the force–time idea to explain how each feature reduces the force on an occupant).

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Forces in a Collision

When a car crashes into a wall or another vehicle, it is brought to rest very suddenly. The people inside are also moving, and unless something stops them they will keep moving forward at the car's original speed until they hit the dashboard, the windscreen or the seat in front — this is why unbelted passengers are thrown forward in a crash.

To stop a moving person you have to change their motion, and changing motion needs a force. The important physics is this: the force needed to stop something depends on how quickly its motion is changed.

• If a person is stopped very suddenly (over a very short time), the force on them is large — large enough to cause serious injury.
• If the same change in motion is spread over a longer time, the force on them is smaller — and a smaller force is much less likely to injure them.

Every car safety feature is designed around this one idea: it makes the occupants lose their motion over as long a time as possible, so that the force on them is as small as possible. (In Topic P2 you learned that this "quantity of motion" is called momentum, $p = mv$p=mv; a crash brings that momentum down to zero, and it is the time over which that happens that controls the force.)

Exam Tip: The heart of every car-safety answer is: for a given change in motion, a longer stopping time means a smaller force. Learn this sentence — it is the reason crumple zones, seat belts and airbags all work.

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Increasing the Stopping Time Reduces the Force

It helps to picture the same crash happening in two different ways. Suppose a passenger of a given mass is moving at a given speed and must be brought to rest.

• Without a safety feature: the passenger keeps moving until they hit a hard surface (the dashboard). A hard surface does not "give", so they are stopped in a tiny fraction of a second — an extremely short stopping time. The force on them is therefore very large, and the injuries are severe.
• With a safety feature: something soft or collapsible extends the stopping — the passenger sinks into an airbag, or the seat belt stretches a little. Now the same passenger, losing the same amount of motion, is brought to rest over a longer time. The force on them is therefore much smaller, and they are far more likely to escape serious injury.

Notice that the passenger has to lose the same amount of motion in both cases — that is fixed by how heavy they are and how fast the car was going. What the safety feature changes is only the time. Spreading the same change of motion over a longer time is what lowers the force.

A familiar everyday example makes the same point. A cricketer catching a fast ball pulls their hands backwards as they catch it, giving the ball a longer time to stop and so reducing the sting on their palms. A high-jumper lands on a thick, soft crash mat rather than bare ground for exactly the same reason: the mat extends the time over which they stop, cutting the force on their body. In a car, the safety features do the same job for the occupants.

Exam Tip: A frequent misconception is that safety features work by "slowing the car down more gently so there is less energy". That is wrong: the change in motion (and the energy that must be removed) is the same. What the features actually do is increase the stopping time, which reduces the force — it is the force, not the energy, that is cut.

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Car Safety Features

Modern cars are engineered so that, in a crash, the occupants lose their motion over as long a time as possible. Each major safety feature works on this same principle.

• Crumple zones are regions at the front (and rear) of the car designed to fold up and crush in a collision. As they crumple, the front of the car takes a longer time to come to rest, so the occupants' change in motion happens over a longer time, reducing the force on them. The crumpling metal also absorbs energy as it deforms.
• Seat belts are made slightly stretchy on purpose. In a crash the belt stretches a little, allowing the wearer to move forward slightly and stop over a longer time than if they had been thrown against the hard dashboard or windscreen — again reducing the force. Seat belts also stop the occupant being thrown out of, or around, the vehicle.
• Airbags inflate rapidly in front of the driver and passenger. The occupant sinks into the airbag as it partly deflates, which increases the time taken for their head and chest to stop, reducing the force on them. The airbag also spreads the force over a larger area of the body, which lowers the pressure and reduces injury.

All three features share the same physics: they extend the time over which the occupant loses their motion, and a longer stopping time means a smaller, less injurious force.

Exam Tip: For any safety feature, the mark-scheme answer is the same: it increases the time over which the change in motion happens, which reduces the force on the occupant. This one sentence works for crumple zones, seat belts, airbags, crash mats and cricket gloves alike.

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Comparing the Safety Features

Although all three work by extending the stopping time, each has its own role, and they work together as a system.

Feature	Where it is	How it reduces the force	Extra benefit
Crumple zone	Front (and rear) of the car body	Folds up, so the car takes longer to stop	Absorbs energy by deforming
Seat belt	Across the occupant	Stretches slightly, so the occupant stops over a longer time	Holds the occupant in the seat
Airbag	In front of the driver/passenger	Occupant sinks into it, extending the stopping time	Spreads the force over a larger area

The seat belt and airbag work best together: the belt holds the occupant roughly in place so that they move into the airbag in a controlled way rather than being flung past it. This is why cars are designed with all these features rather than relying on any single one.

Exam Tip: If asked to compare two safety features, say what they have in common (both increase the stopping time and so reduce the force) as well as how they differ (for example, the airbag also spreads the force over a larger area, while the seat belt also stops the occupant being thrown out). Balanced answers score highest.

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Other Safety Features That Work the Same Way