Understanding Car Crashes: It's Basic Physics

Summary

This educational video explores the physics behind car crashes, emphasizing the importance of understanding Newton's laws of motion, inertia, and conservation of momentum for vehicle safety. Host Griff Jones, a science educator, explains concepts like kinetic energy and crashworthiness through real-world examples and interviews with vehicle safety experts. The video highlights the role of seat belts, crumple zones, and airbags in protecting occupants during collisions, stressing that even with advanced technology, the laws of physics dictate the outcome of a crash.

Takeaways

• 🏎️ High-speed driving can lead to tragic accidents due to the laws of physics, emphasizing the importance of understanding these laws for safety.
• 🧐 Inertia, as described by Newton's first law, plays a critical role in car crashes, affecting both the vehicle and its occupants.
• 💺 Seat belts are crucial as they connect occupants to the vehicle's deceleration during a crash, reducing the impact of inertia.
• 🚗 Newton's second law (F=MA) helps explain the forces involved in car crashes and the importance of crumple zones in absorbing energy.
• 📉 The concept of momentum and its change through impulse is key to understanding the effects of collisions on vehicle occupants.
• 🔁 Impulse is demonstrated practically, showing how spreading force over time can reduce damage, akin to vehicle crumple zones.
• 🚀 The unit 'g' measures the force experienced during rapid deceleration, with high gs in crashes potentially leading to serious injury.
• 🛑 Extending the time of impact, as with airbags and crumple zones, reduces g-forces on occupants and can be lifesaving.
• 🚗💥 In collisions, the conservation of momentum means that the total motion before and after a crash remains the same, impacting the forces felt by occupants.
• 🚕🚗 The mass of vehicles affects crash outcomes, with lighter vehicles and their occupants experiencing greater forces in collisions with heavier vehicles.
• 💥 Kinetic energy, which increases with speed, plays a significant role in the severity of car crashes and the potential for injury.
• 🛠️ Vehicle structural design and crashworthiness are critical for protecting occupants, with modern vehicles evolving to better manage crash forces.

Q & A

What is the main focus of the Insurance Institute for Highway Safety's Vehicle Research Center?

The main focus of the Insurance Institute for Highway Safety's Vehicle Research Center is to assess the crash performance of vehicles by running tests and evaluate new technologies to prevent injuries.

How does the concept of inertia relate to vehicle crashes?

Inertia, the property of matter that resists changes in its state of motion, is crucial in vehicle crashes. It explains why an unbelted occupant continues moving at the vehicle's speed upon impact, leading to potential injuries from striking the interior of the car.

What is the significance of Newton's first law of motion, the law of inertia, in the context of car crashes?

Newton's first law of motion, the law of inertia, is significant in car crashes because it explains why passengers continue moving at the same speed as the vehicle until acted upon by an external force, such as the impact with the steering wheel or windshield during a crash.

Why are seat belts important during a car crash?

Seat belts are important during a car crash because they tie the occupant to the vehicle, allowing them to slow down with the occupant compartment as the vehicle's front end crumples and absorbs crash forces, thus reducing the impact forces on the body.

How does Newton's second law, expressed as F=ma, relate to the forces experienced in a car crash?

Newton's second law, F=ma, relates to car crashes by illustrating the force required to move an object (or in this case, decelerate a vehicle and its occupants) is dependent on the mass of the object and the acceleration (or deceleration) it experiences during the crash.

What is the difference between momentum and impulse in the context of a car crash?

Momentum is the quantity of motion, calculated as the product of an object's mass and velocity, while impulse is the product of force and the time during which the force acts. In a car crash, the change in momentum is achieved through an impulse, such as the impact with a barrier or another vehicle.

Why do airbags and crumple zones in cars help to improve safety during a crash?

Airbags and crumple zones help improve safety during a crash by extending the time of impact, which reduces the force experienced by the occupants. This is based on the principle that a longer time of impact results in lower acceleration forces, reducing the risk of injury.

What is the law of conservation of momentum, and how does it apply to collisions between vehicles?

The law of conservation of momentum states that the total quantity of motion (momentum) in a closed system remains constant. In vehicle collisions, this means that the total momentum before the collision is equal to the total momentum after the collision, regardless of the vehicles' masses and speeds.

How does the mass of a vehicle affect the safety of its occupants in a collision with another vehicle?

In a collision between vehicles of unequal mass, the occupants of the lighter vehicle experience greater acceleration and force due to a larger change in speed, which can lead to more severe injuries. This is because the momentum change is distributed over a shorter time period in the lighter vehicle.

What is kinetic energy, and how does it relate to the severity of car crashes?

Kinetic energy is the energy an object possesses due to its motion, calculated as one half times the mass of the object times the velocity squared. In car crashes, the severity of the crash and the potential for injury are related to the kinetic energy involved, as higher kinetic energy results in greater forces during impact.

Why is it important for a vehicle's structure to remain intact during a crash?

A vehicle's structure, or safety cage, should remain intact during a crash to protect the occupants by containing the crash forces within the front end of the vehicle. If the structure fails and the occupant compartment is compromised, the risk of injury to the occupants significantly increases.

Outlines

00:00

🏎️ Car Crashes and Physics

The paragraph introduces the topic of car crashes and their relation to the laws of physics. Griff Jones, a science education professor, explains how the Insurance Institute for Highway Safety's Vehicle Research Center conducts tests to evaluate vehicle crash performance and new technologies to prevent injuries. It highlights the importance of understanding basic physical laws to comprehend the outcomes of car crashes, such as the role of inertia, which is why seat belts are crucial during a crash. The paragraph sets the stage for exploring the science behind vehicle crashes.

05:01

🔍 Newton's Laws and Crash Forces

This section delves into Newton's laws of motion, particularly the second law, which relates force, mass, and acceleration (F=MA). It explains how these laws apply to car crashes, emphasizing the concept of momentum and impulse. The paragraph uses examples, such as eggs hitting a wall versus a sheet, to illustrate the difference between high force over a short time and lower force over a longer time. It also discusses how the deceleration experienced by a car's occupants during a crash can be lessened by extending the time of impact, which is the principle behind airbags and crumple zones.

10:03

📚 Conservation of Momentum and Crash Dynamics

The paragraph explores the law of conservation of momentum and its application to car crashes. It explains that momentum is a vector quantity and that in a head-on collision, the momenta of two identical cars cancel each other out. The discussion then turns to the implications of collisions between vehicles of different masses, highlighting that the lighter vehicle's occupants experience higher accelerations and forces due to a greater speed change during the crash. The importance of vehicle size and weight in crash protection is also mentioned, with larger, heavier cars offering more protection.

15:03

⚖️ Kinetic Energy and Crashworthiness

This section discusses kinetic energy and its role in car crashes. It explains that the kinetic energy of a moving object is proportional to the square of its speed, which means that even a small increase in speed can lead to a significant increase in kinetic energy and potential for damage. The paragraph also covers the concept of work, which is the application of force over a distance to reduce a car's kinetic energy. The discussion then moves to crashworthiness, which involves the vehicle's structure and restraint systems. The importance of a vehicle's ability to absorb crash energy through its crumple zones and the protection offered by airbags and seat belts are highlighted.

20:05

🔄 Evolving Crash Tests and Safety Improvements

The final paragraph addresses the evolution of crash tests and the ongoing efforts to improve vehicle safety. It discusses how the Insurance Institute for Highway Safety has expanded its testing to include different crash scenarios, such as 25% overlap tests, and the challenges automakers face in designing vehicles that can channel crash energy away from the occupant compartment. The conversation also touches on the need to improve safety for rear-seat passengers, who may not have the same level of protection as front-seat occupants due to a lack of advanced restraint systems. The paragraph concludes with a call to action for viewers to understand the importance of physics in car safety and the impact of their choices on vehicle safety.

Keywords

💡Inertia

Inertia is the property of matter that resists changes in its state of motion. Defined by Newton's first law of motion, it is a fundamental concept in physics that explains why objects at rest tend to stay at rest and objects in motion tend to stay in motion unless acted upon by an external force. In the context of the video, inertia is illustrated by the crash test dummy continuing to move at the original speed until it is stopped by the steering wheel and windshield, emphasizing the importance of seat belts in slowing down occupants with the vehicle during a crash.

💡Crumple Zones

Crumple zones are designed areas in a vehicle's structure that are intended to collapse and absorb energy during a collision. This design helps to protect the vehicle's occupants by reducing the force of impact. The video explains that crumple zones are crucial in managing the force exerted on passengers during a crash, as they allow the vehicle's front end to crumple and absorb the crash forces, thereby reducing the deceleration experienced by the occupants.

💡Kinetic Energy

Kinetic energy is the energy possessed by an object due to its motion. It is calculated as one half of the mass of the object multiplied by the square of its velocity. The video emphasizes that kinetic energy is a critical factor in car crashes because it is directly proportional to the square of the speed. This means that even a small increase in speed can lead to a significant increase in kinetic energy, which in turn can cause more severe damage in a crash.

💡Force

Force is any interaction that, when unopposed, will change the motion of an object. In the video, force is discussed in the context of Newton's second law of motion, expressed as F=ma, where force is equal to mass times acceleration. The video uses force to explain how the front end of a car crushes and absorbs energy during a crash, which slows down the vehicle and illustrates the concept of impulse as force applied over time.

💡Impulse

Impulse is the product of force and the time during which the force acts. It is a measure of change in an object's momentum. The video uses the example of an egg hitting a wall versus a sheet to demonstrate that a smaller force applied over a longer time can have the same impulse but result in less damage, illustrating the importance of extending the time of impact to reduce forces experienced in a crash.

💡Momentum

Momentum is a vector quantity that represents the quantity of motion of an object, calculated as the product of the object's mass and velocity. The video explains that momentum is conserved in collisions, meaning the total momentum before a collision is equal to the total momentum after the collision. This principle is crucial in understanding the dynamics of car crashes and the forces experienced by the occupants.

💡Acceleration

Acceleration is the rate at which an object changes its velocity. It is a vector quantity that has both magnitude and direction. In the video, acceleration is discussed in relation to the forces experienced by occupants during a crash, with higher accelerations resulting in greater forces and potential for injury. The video also mentions 'gs' as a unit of acceleration experienced during high-speed maneuvers or crashes.

💡Crashworthiness

Crashworthiness refers to the ability of a vehicle to protect its occupants during a crash. The video discusses how crashworthiness is determined by various aspects of vehicle design, including the structure, the restraint system, and the effectiveness of the crumple zones. Good crashworthiness is illustrated by a vehicle that maintains the integrity of its occupant compartment while directing the forces of the crash to the crumple zones.

💡Seat Belts

Seat belts are safety restraints designed to secure the vehicle's occupants and reduce the risk of injury during a crash. The video emphasizes the importance of seat belts in managing the forces experienced by occupants during a crash. By securing passengers to the vehicle, seat belts allow them to slow down with the vehicle's occupant compartment, reducing the risk of injury from the forces generated during a collision.

💡Airbags

Airbags are safety devices that inflate rapidly during a collision to cushion and protect vehicle occupants from hitting the vehicle's hard surfaces. The video mentions airbags as part of the vehicle's safety features that work in conjunction with crumple zones and seat belts to extend the time of impact and reduce the forces experienced by occupants during a crash.

Highlights

The video explores the physics behind car crashes, emphasizing the importance of understanding physical laws to prevent injuries.

Inertia, as explained by Newton's first law, plays a crucial role in understanding why passengers continue moving at the original speed during a crash.

The necessity of seat belts is highlighted, as they help passengers slow down with the vehicle during a crash, overcoming inertia.

Newton's second law, F=MA, is used to explain the relationship between crash forces and inertia.

Momentum, defined as the product of an object's mass and velocity, is shown to be a key factor in crash outcomes.

Impulse, the product of force and time, is demonstrated through an egg-cracking experiment to illustrate its effects on objects.

The concept of gs, or acceleration units, is introduced to explain the forces experienced by passengers during a crash.

The importance of extending the time of impact to reduce deceleration forces is discussed, with examples like airbags and crumple zones.

Conservation of momentum is applied to car crashes, showing how it affects occupants in collisions of vehicles of different masses.

The difference between weight and size advantage in car crashes is explained, emphasizing the protection offered by heavier and larger vehicles.

Kinetic energy's role in car crashes is discussed, with the formula KE = 1/2 MV^2 highlighting the significance of speed.

The potential energy and kinetic energy transformation is demonstrated using a pendulum, explaining energy transfer in crashes.

Crashworthiness is defined as the protection a car offers its occupants, involving aspects like structure and restraint systems.

The importance of a vehicle's structure performing well in a crash to protect the occupant compartment is emphasized.

The development of new crash tests by the Insurance Institute for Highway Safety, including a 25% overlap test, is discussed.

The rear seat safety is highlighted as an area of concern, with plans to develop tests to improve rear seat passenger protection.

The video concludes with a call to action for viewers to understand the laws of physics to make informed decisions about vehicle safety.

Transcripts

(upbeat rock music)

- [Announcer] Gentlemen, start your engines.

(engines revving)

(upbeat rock music)

- [Announcer] Green, green, green, green.

(upbeat rock music)

(tires screeching)

(upbeat rock music)

(car crashing)

(upbeat rock music)

(tires screeching)

- These drivers lost control at very high speeds.

The result was tragic for one driver,

and fortunate for the others, but why?

What made the difference between walking away

and being carried away?

The answer can be found in some of the most basic laws

of the physical universe.

(engine revving)

(car crashing)

Hi, I'm Griff Jones.

I'm a science education professor,

and behind me is the Insurance Institute

for Highway Safety's Vehicle Research Center.

It's a fascinating place, where research engineers

assess the crash performance of vehicles by running tests,

and where they evaluate new technologies

to prevent injuries.

When I first came here, I was a high school physics teacher.

What was exciting for me then, and still is today,

is that this is a laboratory of practical applications

in science, technology, engineering, and mathematics.

And because they're set up here to crash cars

and analyze those crashes,

this research center provides the perfect venue

for illustrating the physical laws

that govern the outcome of car crashes.

Even though we made the original version of this video

a number of years ago, it's still relevant

because the laws of physics haven't changed.

So let's go back and explore the basic science

behind vehicle crashes.

Let's learn about car crashes and physics.

(video scratching)

Let's learn about car crashes and physics.

(dummy thudding)

Why'd this dummy get left behind?

It's called inertia, the property of matter

that causes it to resist any change in its state of motion.

Galileo introduced the concept in the late 1500s,

and almost a hundred years later,

Newton used this idea to formulate his first law of motion,

the law of inertia.

It's why the dummy fell off the back of the truck.

It was at rest and it wanted to remain at rest,

that's inertia.

(upbeat piano music)

It's the same property that keeps the china on the table

as you pull the table cloth out from under it.

(dishes clinking)

(triumphant music)

Now what about a body in motion?

Am I a body in motion?

You bet I am.

I'm moving 35 miles per hour, but from one perspective,

it may not look like I'm moving at all

because in relationship to the passenger compartment,

my position isn't changing.

But if you look at me from the outside,

you can see that I'm moving at the same speed

as the vehicle.

In this case, about 35 miles per hour.

And if Newton was right, and he know he was,

I'm going to keep on moving at this same speed

until an external force acts on me.

Now what does this mean to occupants of a moving vehicle?

Watch this.

(car crashing)

See how the car and the crash test dummy

are traveling at the same speed?

Now watch what happens

when the car crashes into the barrier.

The front end of the car is crushing and absorbing energy,

which slows down the rest of the car.

But, the dummy inside keeps on moving at its original speed

until it strikes the steering wheel and windshield.

This is because the dummy is a body in motion

traveling at 35 miles per hour,

and remains traveling 35 miles per hour

in the same direction until acted upon by an outside force.

In this case, it's the impact of the steering wheel

and windshield that applies force

that overcomes the dummy's inertia.

Inertia is one reason that seat belts are so important.

Inertia is one reason that you wanna be tied to the vehicle.

during a crash.

If you're wearing your seat belt,

you slow down with the occupant compartment

as the vehicle's front end does its job

of crumpling and absorbing crash forces.

Later, we'll talk about how some vehicles' front ends,

or crumple zones,

do a better job of absorbing crash forces than others.

(car crashing)

But for now, let's get back to Newton.

He explained the relationship

between crash forces and inertia in his second law,

and the way it's often expressed is F equals MA.

The force F is what's needed to move the mass M

with the acceleration A.

Newton wrote it this way.

It's the same thing.

Acceleration is the rate at which the velocity changes.

But if I multiply each side of the equation by T,

I get force times time equals

mass times a change in velocity.

When Newton described the relationship

between force and inertia,

he actually spoke in terms of changing momentum

with an impulse.

What do these terms mean?

(upbeat rock music)

Momentum is inertia in motion.

Newton defined it as the quantity of motion.

(upbeat rock music)

It's the product of an object's mass, its inertia,

and its velocity or speed.

Which has more momentum,

an 80,000 pound big rig traveling two miles per hour

or a 4,000 pound SUV traveling 40 miles per hour?

The answer is they both have the same momentum.

Here's the formula.

P is for momentum.

I don't know why they use P, they just do.

Equals M is for mass, and V is for velocity.

P equals MV, that's momentum.

(upbeat rock music)

And what is it that changes an object's momentum?

It's called an impulse.

It's the product

of force and the time during which the force acts.

Impulse equals force times time.

Here's my favorite demonstration of impulse.

I have two eggs, same mass.

I'm going to try to throw each egg with the same velocity.

That means they have the same momentum.

(upbeat music)

If the impulses were equal,

why do we have such dramatically different results?

The wall applies a big stopping force over a short time.

The sheet applies a smaller stopping force

over a longer time period.

My students say the sheet has more give to it.

Both stop the egg,

both decelerate the egg's momentum to zero,

but it takes a smaller force to reduce the egg's momentum

over a longer time.

In fact, so much smaller that it doesn't even crack

the egg's shell.

Now let's relate this to automobiles.

Both of these cars have the same mass

and both are traveling at the same speed, 30 miles per hour.

Like the eggs, they have equal momentum.

As a result, it will take equal impulses

to reduce their momentum to zero.

One car will stop by panic braking

and the other by normal braking.

If both drivers are belted,

so they decelerate with their vehicles,

the driver of the car on the bottom

will experience more force than the driver on top.

This is because if the impulses must be equal

to decelerate each car's momentum to zero,

the driver that stops in less time or distance

must experience a larger force and the higher deceleration.

(upbeat rock music)

A g is a standard unit of acceleration or deceleration.

People often refer to gs as forces, but they're not.

Fighter pilots can feel as many as nine gs

when accelerating during extreme maneuvers,

and astronauts have felt as many as 11.

(car crashing)

People in serious car crashes experience even higher gs,

and this can cause injury.

(car crashing)

Now consider what happens when a car traveling

30 miles per hour hits a rigid wall,

which shortens the stopping time or distance

much more than panic braking.

Let's again assume the driver is belted

and decelerates with the passenger compartment.

And let's also assume the car's front end

crushes one foot with uniform deceleration

of the passenger compartment throughout the crash.

In this crash, the driver would experience 30 gs.

However, if the vehicle's front end was less stiff,

so it crushed two feet instead of one,

the deceleration would be cut in half to 15 gs.

This is because the crush distance,

or the time the force is acting on the driver, is doubled.

Extending the time of impact is the basis

for many of the ideas about keeping people safe in crashes.

It's the reason for airbags and crumple zones

in the vehicles you drive.

It's the reason for crash cushions

and breakaway utility poles on a highway.

And it's the answer to the question I posed

at the beginning of this film.

This driver survived the crash because his deceleration

from high speed took place over a number of seconds.

This driver decelerated in a small fraction of a second

and experienced forces that are often unsurvivable.

Up to now, we've been looking at single vehicle crashes,

but if we look at two or more objects colliding,

we have to use another one of Newton's laws

to explain the result.

Even though the first cars wouldn't appear on the roads

for over 200 years, collisions were an active topic

of physics research in Newton's day.

Back in 1662, Newton and his buddies formed

one of the first international science clubs.

They called it the Royal Society of London

for Improving Natural Knowledge.

One of the first experiments they did was to test

Newton's theories on collisions using a device like this.

What do you think's gonna happen when I release this ball

and it collides with the others?

(balls clinking)

Let's try two.

(balls clinking)

It's as if something about the collision

is remembered or saved.

(balls clinking)

Newton theorized that the total quantity of motion,

which he called momentum, doesn't change, it's conserved.

This became known as a law of conservation of momentum

and it's one of the cornerstone principals

of modern physics.

(balls clinking)

Before we apply this to crashing cars,

we need to know something else about momentum.

It has a directional property.

So we call momentum a vector quantity.

This means if identical cars traveling 30 miles per hour

collide head on, their momenta cancel each other.

(upbeat music)

Inside the passenger compartment of each car,

the occupants would experience the same decelerations

from 30 miles per hour to zero.

(upbeat music)

The dynamics of this crash would be the same

as a single vehicle crash into a rigid barrier.

(upbeat music)

What conservation of momentum tells us

about collisions of vehicles of different masses

has important implications for the occupants

of both the heavier and lighter vehicle.

(upbeat music)

In a collision of two cars of unequal mass,

the more massive car would drive the passenger compartment

of the less massive car backward during the crash,

causing a greater speed change

in the lighter car than the heavier car.

These different speed changes occur during the same time,

so the occupants of the lighter car would experience

much higher accelerations, hence much higher forces

than the occupant of the heavier car.

This is one reason why lighter, smaller cars

offer less protection to the occupants

than larger, heavier cars.

There's a difference between weight and size advantage

in car crashes.

Size helps you in all kinds of crashes.

(cars crashing)

Weight is primarily an advantage

in a crash with another vehicle.

(cars crashing)

(classical music)

Newton was a pretty brilliant guy.

The laws of motion he advanced over 300 years ago

are still used today to explain the dynamics

of modern day events, like car crashes.

(classical music)

But even Newton failed to recognize the existence of energy.

Even though it's all around us,

energy is tough to conceptualize.

Scientists have had difficulty defining energy

because it exists in so many different forms.

It's usually defined as the ability to do work,

or as one of my students says,

"It's the stuff that makes things move."

Energy comes in many forms.

There's radiant, electrical, chemical, thermal

and nuclear energy.

In relating the concept of energy to car crashes though,

we are mostly concerned

with motion-related energy, kinetic energy.

(upbeat music)

Moving objects have kinetic energy.

A baseball thrown to a batter,

a diver heading toward the water,

an airplane flying through the sky,

a car traveling down the highway all have kinetic energy.

But energy doesn't have to involve motion.

An object can have stored energy due to its position

or its condition.

This is a device that delivers a force

to a crash dummy's chest to test the stiffness of the ribs.

(pendulum banging)

The force is a result of the kinetic energy

being transferred from the pendulum to the dummy's chest.

As the pendulum sits at its ready position,

its potential energy is equal

to its kinetic energy at impact.

When it is released and begins traveling

towards the dummy's chest,

the potential energy transforms into kinetic energy.

If we freeze the pendulum halfway,

what is its potential versus kinetic energy?

They are equal.

When has the pendulum reached its maximum kinetic energy?

Here, at the bottom of its swing.

(pendulum banging)

The amount of kinetic energy an object has

depends upon its mass and velocity.

The greater the mass, the greater the kinetic energy.

The greater the velocity, the greater the kinetic energy.

The formula that we use to calculate kinetic energy

looks like this.

KE, that's kinetic energy,

equals one half MV squared.

That's the velocity multiplied by itself.

And if you do the math,

you'll see why speed is such a critical factor

in the outcome of a car collision.

The kinetic energy is proportional

to the square of the speed.

So if we double the speed,

we quadruple the amount of energy in a car collision.

And energy is the stuff that has potential to do damage.

(upbeat music)

The connection between kinetic energy and force

is that in order to reduce a car's kinetic energy,

a decelerating force must be applied over a distance.

That's work.

To shed four times as much kinetic energy

requires either a decelerating force

that's four times as great,

or four times as much crush distance,

or a combination of the two.

(cars crashing)

The rapid transfer of kinetic energy

is the cause of crash injuries.

So managing kinetic energy is what keeping people safe

in car crashes is all about.

Brian O'Neill is the President

of the Insurance Institute for Highway Safety.

(car crashing)

- [Griff] That's incredible.

- So one of the things we do, we put grease paint on the--

- [Griff] He runs the Vehicle Research Center

and is one of the foremost experts in the world

on vehicle safety.

- Where the dummy hits.

We use the term crash worthiness to describe the protection

a car offers its occupants during a crash.

Now, crash worthiness is a complicated concept

because it involves many aspects of the open design.

The structure, the restraint system,

it all adds up to the single term we use, crashworthiness.

We use this stripped down body to illustrate

the concepts of good and poor structural designs

for modern crashworthiness.

- Brian, why is it important for the vehicle's structure

to perform well in a crash?

- Well this is what's left of the body and structure

of a car that was in a crash

and we use this to illustrate the point.

Basically, we want the occupant compartment,

or the safety cage, to remain intact.

We don't want any damage or intrusion

into this part of the vehicle during the crash.

We want all of the damage of the crash

confined to the front end.

- So even though all this metal looks the same,

it's actually different.

The green metal's intended to crumple

to give in the collision.

- If we can crumple the front end of the car

without allowing any damage to the occupant compartment,

then the people inside can be protected

against serious injury.

Basically, we want the front end to be buckling

during the crash so that the occupant compartment

is slowed down at a gentler rate.

- Right, kinda like jumping off of a step

and keeping your knees straight and landing on the floor

versus bending your knees when you land.

- Exactly the same concept.

So this is a vehicle that did well

because there's very little intrusion

anywhere in the occupant compartment.

These elements here, even though they're strong enough

to hold an engine and suspension,

actually buckled and crushed

just like they're designed to do.

So, the damage is confined to the front end.

We look at a vehicle like this,

and this is an example of a very poor safety cage.

This vehicle was in a 40 mile an hour crash,

and as you can see, the occupant compartment has collapsed.

It's been driven backwards.

As a result, the driver's space has been greatly reduced.

So someone sitting in this vehicle

is obviously at a high risk of injury.

- So even if the restraint systems do function properly,

the airbag, the seat belt,

the person still is in great danger.

- This person in this vehicle,

even with a belt system and airbag,

is at significant risk of injury

because the compartment is collapsing.

- So it's analogous to shipping a box of china.

You can have all the best packing in the world

around the china, but if the box is weak,

you're gonna break the china.

- When the safety cage collapses,

you're gonna have injuries to the occupants.

So this is an example of poor crashworthiness.

But this vehicle was in the same crash,

40 mile an hour offset crash,

and you can see now the safety cage has remained intact.

There's very little intrusion anywhere.

The damage is confined to the crumple zone of the vehicle.

This is the way it should be.

A person in a crash like this,

wearing their seat belt and protected by the airbag,

can walk away from the crash with no injury.

- Right, if I stand over here and I just look

towards the rear of the car and I ignore the airbag,

this doesn't even look like it's been in a crash.

- That's right, this is good performance,

good crashworthiness.

- In our shipping box analogy,

this is an example of a strong box.

- That's right, the people in this box will be protected.

(car crashing)

- [Griff] Since we first made this film,

automakers have responded,

and now all vehicles perform well in the 40% overlap test.

But the Institute and its current President, David Harkey,

have continued to advance frontal crashworthiness testing.

- So these are the vehicles that you and Brian

were talking about.

Every vehicle passes this test with no problem now

and gets a good rating.

One of the things that we started looking at

was why are still having so many fatalities

in frontal crashes, right?

Even in good performing vehicles.

And one of the things that we determined

is that not all of those frontal crashes have a 40% overlap.

There are many instances where the amount of overlap

is much less than that.

And so, what we did was we created a test

where the amount of overlap was 25%.

- [Griff] It's like the side of the car

is being sheared away.

- It really is, and so you can see from the two vehicles

that we have here,

this vehicle obviously got a poor rating.

Everything was pushed back into the occupant compartment.

- It's tough for the automakers to address,

but looks like they did it here.

- They strengthened the structure here.

They also have to figure out how to design the suspension,

the wheel so that it doesn't push back into that firewall.

What makes this test so hard is that all of that energy

is occurring outside of the primary structural frame rail.

- [Griff] Right, so they're missing the frame rail,

which is the beginning of the crumple zone.

- [David] Exactly.

- It's quite an engineering challenge

to take all of that energy and still channel it in a way

so that it doesn't intrude on the occupant compartment.

Is this 25% overlap test still evolving?

- [David] The biggest change with this test

is we've added a very similar test

for the other side of the vehicle.

Now we've added a crash test dummy in the passenger seat

to be able to look for injury metrics

on that side of the vehicle as well.

- Are you tweaking these front crash tests anymore?

- There's nothing on the horizon in terms of the front seat

right now, but we are looking at real world data,

and we're concerned about the rear seat,

and that's where we're going next.

- [Griff] It turns out in some vehicles,

passengers buckled up in the rear seat

are more likely to be injured than those belted

in the front seat.

This was a surprise to me.

I'd always heard back seat's always gonna be

a safer place to be, but it's not the case now.

- Well, the important thing here.

It's not that the rear seat has become less safe,

it's just that our focus has been on the front seat

for so long now.

That's where the automakers have really looked

to put interventions in the vehicle.

- So what does the front seat have

that the back seat doesn't?

- The front seat has airbags that deploy

to protect the passengers in the event of a frontal crash.

It also has, in the belt system,

two specific features nowadays,

crash tensioners and force limiters.

Both of these act in the event of a crash.

The crash tensioner to pull the belt tight,

and then the force limiter to let the webbing of the belt

spool out just slightly to limit those forces on the chest

during the crash.

- So, just like the crumple zone and the airbag,

physics is the same.

You're increasing that time of impact,

but just within the seat belt mechanism.

In the rear seat, they don't have those things?

- There are very few vehicles now in production

that have those two components built into the belt system.

- So your goal is to figure out what's the best test

to show what's happening to the passenger in the rear seat,

and once you've developed that test,

then it's up to the automakers to try to figure out a way

to make it safer.

- [David] That's correct.

- And even though you're changing the test,

the physics is still the same.

- The physics is still the same.

We're just moving to a different part of the vehicle.

(car crashing)

- I'm always looking for ways

to relate the physics that I teach to the real world

that students experience, and nothing is more relevant

than traveling in an automobile.

You probably do it everyday.

Even with advances in crash avoidance technology,

crashes still occur.

I hope that makes the message of this film

important to each and every one of you.

I've always believed that if a person truly understands

the laws of physics,

that person would never ride in a motor vehicle unbelted.

And now that you've had a chance to learn

some of the finer points of the physics of car crashes,

I hope you agree.

(upbeat music)

I also hope you've learned why some of the choices you make

about the type of car you drive

and the kind of driving you do

makes a difference on whether you survive on the highway.

Remember, even the best protected race car drivers

don't survive very high speed crashes.

The bottom line is still the same

as when we first made this video.

The dynamics of a motor vehicle crash,

what happens to your car and you,

is determined by hard science.

You can't argue with the laws of physics.

(upbeat music)