Road crashes are one of the top killers in our country. More than 1.5 lakh of our fellow countrymen lose their lives every year. In addition, many people sustain debilitating injuries.
All of us face this risk. We just don’t realise it. The best way to solve a problem is to understand it first. And just like everything else in the universe, road crashes are also governed by the laws of physics. That is why understanding the physics behind road crashes is beneficial to keep ourselves safe.
The purpose of this article is to understand how the laws of physics influence road crash and injuries. Don’t worry if you don’t like physics (I don’t like it either!) I have kept this article simple to understand.
The physics behind road crashes
There are multiple principles that govern road crashes. But, we won’t go into the details and only focus on the ones that will benefit us the most.
Inertia
Let us start with an example. Look at this video from an immensely popular and funny cartoon series, The Roadrunner Show.
The coyote definitely goes through a lot of pain in this video. But, have you wondered why the coyote stops only after hitting the mountain wall?
The reason is a physical property known as “Inertia”.
Inertia is an object’s property to remain at rest or in motion until a sufficient external force is applied. Inertia is a by-product of Newton’s First Law of Motion. Here is how inertia applies to the video…
- The rubber band springs the coyote behind the roadrunner at great speed.
- However, just as the coyote is about to catch the roadrunner, the road curves and the roadrunner stops by braking.
- But, there is nothing to stop the coyote!
- He continues traveling forward until he hits head-on painfully into the mountain!
- You may have noticed that the coyote also hits many small rocks and signboards. But these don’t provide the necessary force to stop him.
Inertia in road crashes
We experience inertia every time we are in a vehicle. Any occupant inside or on a vehicle has the same inertia as the vehicle. A restrained occupant will experience the same sudden change in motion that the vehicle experiences. An unrestrained occupant will continue to move with the vehicle’s initial inertia.
Inertia acts the same way in road crashes. Let us understand this through an example.
Assume that you are driving a car and hit another car traveling in the front.
- The impact will stop both you and your car. If you are belted, the seat belt will restrain you to the car and stop you from being thrown forward.
- However, things will be different if you are not belted. Your car will stop on impact. But, you will keep on moving forward till you are stopped by a strong object such as the steering wheel or the windshield.
This is why seat belts are perhaps the most important of all car features. The seat belt keeps you restrained to the car and gives your car a chance of protecting you. Otherwise, you are at the mercy of the laws of physics.
You can also refer to this sample crash test to understand how belted and unbelted occupants move inside a vehicle.
Inertia on two-wheelers
Intertial effects are much more severe in two-wheelers. There is no equipment that restrains you to the two-wheeler in a crash. You will always be thrown off the vehicle. That is why you need additional protection such as a helmet and gear.
Impulse
A road crash changes a vehicle’s inertia. A crash will either stop a moving vehicle or move a stopped vehicle.
However, the crash severity is determined by its Impulse.
Let us understand Impulse through an example of a car hitting a wall.
Every moving car has a mass and velocity. When the car hits a wall, the wall generates a resistive force to stop the car. The crash severity is decided by two factors:
- The strength of the resistive force. The higher the force, the more is the severity.
- The time it takes for the vehicle to stop. The impact severity is higher if the vehicle stops quickly.
The combination of the resistive force and crash duration is known as Impulse. The effect of impulse can be best understood through this memorable quote:
As crash force is harder to control, vehicle safety design focuses on increasing the crash duration. The slower you impact, the lesser the injury severity. Seatbelts, Airbags, Padded Helmets/jackets, and Roadside barriers all try to increase the crash duration.
Momentum
Momentum is the product of a vehicle’s mass and velocity. The higher the momentum of a vehicle, the higher it is to stop. Momentum is also directly proportional to Impulse.
Momentum plays an important role in incompatible crashes i.e. crashes between objects of different masses.
Let us consider a crash between a car and a truck. A standard truck can weigh from 15 times to even 40 times that of a standard car! To stop a truck, a car will have to travel at least 15 times faster than the truck! It is no wonder that the car will bear most of the impact forces in such crashes.
Momentum plays an important role even in crashes between cars. A 2-tonne SUV is twice as heavy as a normal hatchback or sedan. The smaller car will need to be twice as fast just to be able to stop the SUV. That is why you will find smaller cars facing the brunt of the impact, irrespective of their NCAP rating.
Remember: NCAP ratings are to be compared between vehicles of a similar mass and not against all vehicles. A 5-star rated hatchback will give a good performance in crashes with vehicles of similar weight. Not against heavier SUVs and definitely not against much heavier pickups, trucks, and buses. Unfortunately, this is another limitation of NCAP ratings that you need to know.
Kinetic Energy
A moving vehicle also has Kinetic Energy. Kinetic Energy is a product of the vehicle’s mass and velocity. Every vehicle loses its kinetic energy in crashes. Some energy is lost in sound and light. However, most of the energy is absorbed by the vehicle itself. This can be very dangerous for the occupants. A safely designed vehicle minimizes kinetic energy transfer to the occupants.
How Vehicle Engineering deals with the Laws of Physics
The physical properties we saw above do not act in isolation! Every crash is governed by Inertia, Impulse, and the laws of Conservation of Energy and Momentum, among various other principles. Every safety system needs to deal with all these principles.
Through rigorous R&D, vehicles and road safety tools are designed to handle the different physical laws governing a road crash. Here is how:
- Seat belts restrain you to the car. This makes it easier for the car to protect you in a crash. An unrestrained occupant is at the mercy of the laws of physics.
- Cars have crumple zones around the passenger cabin. These crumple zones absorb the kinetic energy of a crash. By absorbing this energy, crumple zones achieve two goals: crumple zones prevent the kinetic energy from being transferred to the occupants, and the crumple zones increase the time of the accident (Impulse). You can learn more about crumple zones and judging a car’s build quality in this article.
- Airbags provide an additional cushioning effect to increase the crash duration (Impulse), thereby reducing the crash severity. However, an airbag will not provide the same safety if you do not wear a seat belt.
- Good quality helmets have inner padding that absorbs the kinetic energy of the impact. This reduces the force on the skull. Cycling helmets have MIPS technology to handle rotational impact forces.
- Child Car seats have a special design to protect children as seat belts are not effective for children.
You cannot escape the laws of physics
Most of us wear helmets and seat belts only to avoid punishment by the law. Given the chance, many of us do not take these safety measures.
The laws of physics act no matter who you are or what “contacts” you have. You may pay off the police to avoid punishment for not wearing a seat belt or helmet.
You cannot bribe the laws of physics. It is best to obey them and take safety precautions. After all, we can only work on the things that are in our control.
Last Updated on August 13, 2022 by RSG