(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.
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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)
