Newton’s Third Law states that:

To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions. (Wikipedia)

In a nutshell, for every action force, there is a reaction force with equal magnitude in the opposite direction. Newton’s third law can be observed every day in anyone’s life, however, many of us don’t realize this. One example is when we are walking. Our legs push down on the ground, the opposite reaction making us walk forward. There are so many examples that display Newton’s Third Law of Motion that I could go on forever.

However, the scenario that I think best exemplifies Newton’s Third Law is a car crash.

Luckily there’s nobody inside!

How does the third law relate to a car crash?

In a car crash, the action force(s) are the cars colliding with each other. The reaction force is the force sent back due to the collision, which is what causes the damage to the car. When two cars are headed straight at each other, they are travelling in opposing directions. When they finally collide, if they apply the same amount of force, they will experience a reaction of equal magnitude. This causes the destruction of the front of both cars. Since FORCE IS EQUAL TO MASS MULTIPLIED BY ACCELERATION (F = ma), a car with a much greater mass will likely do better or even survive in a car crash compared to a car with a smaller mass, even if the two vehicles were travelling at the same acceleration.

Unfortunately, there is no specific formula that pertains to Newton’s Third Law, as it is a theoretical law that can only be visually seen. Luckily, Newton’s Third Law is very easy to see since the reaction force is evident right after the action force.

Before being released to the market, new car models must be tested in a crash facility. There, they undergo and must pass safety tests, which involve the infamous crash test dummy in a car speeding head on at a barrier. Here is a video of a crash test:

This video shows that as the car is speeding up toward the wall, it’s acceleration increases, while mass stays constant. This causes the total force to increase. When the car makes contact with the barrier, there is reaction of equal magnitude that arises from the car making contact with the barrier. The reaction force is easily visible in a car crash, which is why it is such a great way to show how Newton’s Third Law can be applied to our lives.

How do the developers determine the safety of the car?

The developers look at the aftermath of the test. With these results, they can determine whether or not the mass of the car should be increased, decreased, or if more safety features should be included, and conclude with the overall safety of the car. Every car has a “crumple zone”, an area of the car that is designed specifically to absorb the force of impact in collisions so that the force of the impact is not transferred to the passengers.

Which cars have the most reliable crumple zones?

Let’s start with the cars with the least reliable crumple zones. Race cars have the least reliable crumple zones because of their shape, lower mass, and higher rates of acceleration. SUV’s tend to have greater reliable crumple zones because they have a greater mass and are able to absorb more kinetic energy.

As far as materials are concerned, the materials used in cars does play a role in protection. For example, carbon fibre is flexible, light weight, and very durable.

Here’s a couple more scenarios where Newton’s Third Law is evident:

Today, many cars have safety features that prevent accidents. These features come into play before the accident, rather than after the collision occurs. They tend to use proximity sensors that detect the distance of other cars from the driver’s car. There are also safety indicators that tell the passengers to buckle their seatbelts, etc. So the new technological safety features don’t use Newton’s Third Law.

In what other ways do you think Newton’s Third Law can be shown in our everyday lives? Leave it in the comment section