Understanding Car Crashes: It's Basic Physics

Summary

The video script explores the physics behind car crashes, emphasizing the importance of understanding basic physical laws like inertia, momentum, and kinetic energy. It delves into the role of vehicle design, crumple zones, and safety features like seatbelts and airbags in mitigating crash forces. The script uses real-world examples and analogies to explain how these principles apply to vehicle safety, urging viewers to appreciate the significance of physics in ensuring their survival on the road.

Takeaways

• 🏎️ High-speed crashes can have tragic outcomes, emphasizing the importance of understanding the physics behind vehicle safety.
• 🧐 Inertia is a fundamental concept in physics that explains why crash test dummies and passengers continue moving at the original speed during a crash until an external force acts upon them.
• 💺 Seatbelts are crucial as they tie occupants to the vehicle, allowing them to slow down with the occupant compartment during a crash.
• 📐 Newton's laws of motion, particularly the laws of inertia and the relationship between force, mass, and acceleration, are key to understanding the dynamics of car crashes.
• 🔍 The crash performance of vehicles is evaluated through tests that assess how effectively crumple zones and other safety features absorb and distribute crash forces.
• 🚗 Modern vehicles are designed with crumple zones that absorb energy during a crash, reducing the forces transferred to the occupants.
• 🛑 Impulse, the product of force and the time over which it acts, explains why extending the time of impact can reduce the force experienced during a crash, such as with airbags and crumple zones.
• 📉 The concept of momentum, mass in motion, and its conservation during collisions is important for understanding the effects on occupants of different vehicles in a crash.
• 🚔 In a collision between vehicles of unequal mass, the lighter vehicle and its occupants experience a greater change in speed and higher acceleration, increasing the risk of injury.
• ⚖️ The difference between weight and size advantage in car crashes is highlighted, with larger, heavier cars generally offering more protection to occupants.
• 🔋 Energy, particularly kinetic energy, plays a significant role in car crashes. The potential for injury increases with the amount of kinetic energy that must be dissipated during a crash.

Q & A

What is the main focus of the video script?

The main focus of the video script is to explain the physics behind car crashes, including concepts like inertia, momentum, kinetic energy, and crashworthiness.

Who is Griff Jones and what is his role in the video?

Griff Jones is a high school physics teacher who guides the viewer through the principles of physics as they relate to car crashes, using the Vehicle Research Center as a backdrop for his explanations.

What is inertia and how does it relate to car crashes?

Inertia is the property of matter that causes it to resist changes in its state of motion. In a car crash, inertia is why an unrestrained occupant continues moving at the original speed of the vehicle until acted upon by an external force, such as the steering wheel or windshield.

Why are seatbelts important during a car crash?

Seatbelts are important 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 overcoming the occupant's inertia.

What does Newton's Second Law of Motion tell us about car crashes?

Newton's Second Law, often expressed as F=ma, tells us that the force needed to move an object is equal to the mass of the object multiplied by its acceleration. In car crashes, this law helps explain the relationship between crash forces and inertia.

What is momentum and how is it calculated?

Momentum is the quantity of motion, which is the product of an object's mass and its velocity. It is calculated using the formula p=mv, where 'p' is momentum, 'm' is mass, and 'v' is velocity.

How does the concept of impulse relate to the deceleration of a vehicle during a crash?

Impulse is the product of force and the time during which the force acts. In a car crash, extending the time of impact (and thus the deceleration) reduces the force experienced by the occupants, which can prevent injury.

Why do airbags and crumple zones exist in vehicles?

Airbags and crumple zones exist to extend the time of impact during a crash, which reduces the force experienced by the occupants. This is based on the principle that a longer crush distance or time results in a lower deceleration rate.

What is the significance of Newton's Law of Conservation of Momentum in car crashes?

Newton's Law of Conservation of Momentum states that the total quantity of motion (momentum) in a closed system remains constant. In car crashes, this law helps explain why occupants of lighter vehicles experience greater forces in collisions with heavier vehicles.

How does kinetic energy play a role in the outcome of a car collision?

Kinetic energy, which depends on an object's mass and the square of its velocity, plays a critical role in car collisions. The greater the kinetic energy, the more severe the potential damage in a crash, as energy is what has the potential to do damage.

What is crashworthiness and why is it important?

Crashworthiness refers to the ability of a vehicle to protect its occupants during a crash. It involves many aspects of vehicle design, including the structure and restraint systems, and is crucial for occupant safety.

What does the future hold for crashworthiness according to Brian O'Neill?

According to Brian O'Neill, while frontal crashworthiness has improved, there is a need to also focus on other crash modes, particularly side-impact crashes, to further enhance vehicle safety.

Outlines

00:00

🏎️ High-Speed Crash Dynamics

The script opens with a dramatic scene of car crashes and introduces the fundamental laws of physics that govern these events. Griff Jones, a high school physics teacher, guides the viewer through the Vehicle Research Center, highlighting the importance of understanding basic physical laws to prevent injuries. He explains concepts like inertia, derived from Newton's first law of motion, and its role in car crashes. The script emphasizes the significance of seatbelts in managing the effects of inertia during a crash.

05:01

📚 Newton's Laws and Crash Forces

This section delves deeper into Newton's laws, particularly the second law expressed as F=ma, to explain the relationship between force, mass, and acceleration in the context of car crashes. Momentum, defined as the product of mass and velocity, is introduced as a key factor in collision outcomes. The script uses the example of two eggs thrown with the same velocity but impacting different surfaces to illustrate the concept of impulse, which is the product of force and the time over which it acts. The discussion highlights how longer stopping times or distances reduce the force experienced during a crash, linking this to safety features like airbags and crumple zones.

10:02

💥 Collisions and Conservation of Momentum

The script explores Newton's theories on collisions and the law of conservation of momentum, which states that the total quantity of motion (momentum) remains constant in a closed system. It explains how momentum, a vector quantity with both magnitude and direction, affects the outcomes of collisions, especially between vehicles of different masses. The discussion emphasizes the higher forces experienced by occupants of lighter vehicles in collisions with heavier ones, underscoring the importance of vehicle weight and size in crash safety.

15:03

🚗 Energy Transfer in Car Crashes

This section focuses on the concept of energy, particularly kinetic energy, which is crucial in understanding the dynamics of car crashes. The script explains that kinetic energy depends on an object's mass and velocity, with the formula KE = 1/2 mv^2 highlighting the significance of speed in crash outcomes. The discussion moves to potential energy and how it transforms into kinetic energy during a collision, such as in a pendulum test on a crash dummy. The script underscores the importance of managing kinetic energy to ensure safety in car crashes.

20:05

🔍 Vehicle Safety and Crashworthiness

The final section discusses the concept of 'crashworthiness,' which encompasses various aspects of vehicle design to protect occupants during a crash. The script contrasts good and poor structural designs in vehicles, emphasizing the importance of maintaining the integrity of the occupant compartment while directing crash damage to the front end. The discussion includes the role of crumple zones, airbags, and seatbelts in managing the forces experienced by occupants during a crash. The script also touches on the challenges of side-impact crashes and the engineering solutions to provide additional protection in such scenarios.

🛣️ Real-World Physics and Vehicle Safety

In the concluding section, Griff Jones reflects on the importance of understanding the physics behind car crashes and how it relates to real-world driving. He emphasizes the relevance of these concepts to everyday life and the importance of making informed choices about vehicle type and driving behavior. The script concludes with a strong message about the inevitability of the laws of physics in determining the outcomes of car crashes, urging viewers to recognize the importance of safety measures and the limitations of even the most advanced protective technologies in high-speed collisions.

Keywords

💡Inertia

Inertia is the property of matter that causes it to resist changes in its state of motion. As defined by Newton's first law of motion, it's the tendency of objects to keep moving in a straight line at constant speed unless acted upon by an external force. In the video, inertia is illustrated by the dummy falling off the truck because it wanted to remain at rest, and by the china staying on the table when the tablecloth is pulled out from under it.

💡Velocity

Velocity refers to the speed of an object in a specific direction. It's a vector quantity, meaning it has both magnitude and direction. In the context of the video, velocity is crucial in determining the momentum and kinetic energy of a moving vehicle. The script mentions Griff Jones moving at 35 miles per hour as an example of velocity.

💡Momentum

Momentum is defined as the product of an object's mass and its velocity. It is a vector quantity, indicating that it has both size and direction. In the video, momentum is discussed in relation to vehicle crashes, explaining that a vehicle's momentum must be reduced to zero during a crash, which requires an impulse.

💡Impulse

Impulse is the product of the force applied to an object and the time for which it is applied. It is what changes an object's momentum. The video uses the example of two eggs being thrown with the same velocity but hitting different surfaces (a wall and a sheet) to demonstrate how impulse affects the outcome of a collision.

💡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 crashes, where the force applied by the steering wheel and windshield overcomes the dummy's inertia. It's also related to the concept of deceleration during a crash.

💡Deceleration

Deceleration is the reduction of an object's velocity, typically measured in 'g' units. It is a vector quantity, having both magnitude and direction. The video explains that in a crash, the deceleration experienced by a vehicle's occupants can lead to injury, with higher 'g' forces indicating more severe impacts.

💡Crashworthiness

Crashworthiness refers to the ability of a vehicle to protect its occupants during a crash. It encompasses the vehicle's structure, restraint systems, and other safety features. The video discusses how good crashworthiness involves the vehicle's structure remaining intact while the crumple zones absorb the crash energy.

💡Crumple Zones

Crumple zones are part of a vehicle's design intended to absorb the energy of a collision gradually and in a controlled manner. They are designed to crumple in a predictable way, spreading the impact forces out over time and reducing the acceleration (g-forces) on the vehicle's occupants. The video mentions crumple zones as a key part of a vehicle's safety design.

💡Kinetic Energy

Kinetic energy is the energy an object possesses due to its motion. It is calculated as one-half the product of the object's mass and the square of its velocity (KE = 0.5 * m * v^2). The video emphasizes that kinetic energy increases with the square of the speed, meaning that even a small increase in speed can lead to a large increase in kinetic energy and potential for damage in a crash.

💡Potential Energy

Potential energy is the stored energy of an object due to its position or condition. In the video, potential energy is discussed in the context of a pendulum, which has potential energy at its highest point. This energy is converted into kinetic energy as the pendulum swings down, impacting a crash test dummy.

💡Conservation of Momentum

The law of conservation of momentum states that the total momentum of a closed system of objects remains constant, unless acted upon by an external force. In the video, this principle is used to explain what happens in collisions between vehicles, emphasizing that momentum is conserved and the total quantity of motion does not change.

Highlights

Gentlemen, start your engines!

Drivers lost control at high speeds with tragic results.

The difference between walking away and being carried away is rooted in the laws of physics.

Griff Jones teaches high school physics and explores vehicle crash performance at the Insurance Institute for Highway Safety's Vehicle Research Center.

Inertia, the property of matter that resists changes in motion, is key to understanding vehicle crashes.

Newton's First Law of Motion, the Law of Inertia, explains why a dummy falls off the back of a truck.

A body in motion, like a moving vehicle, will continue to move at the same speed until acted upon by an external force.

Seatbelts are crucial as they tie you to the vehicle's deceleration during a crash.

Newton's Second Law, F=ma, explains the relationship between crash forces and inertia.

Momentum, the product of an object's mass and velocity, is a key factor in car crashes.

Impulse, the product of force and the time over which it acts, can be demonstrated with eggs hitting different surfaces.

Cars with equal mass and speed require equal impulses to stop, but the time over which they stop affects the force experienced.

The concept of 'g's, or acceleration, is important for understanding the forces in a car crash.

Crash cushions and crumple zones extend the time of impact to reduce forces on occupants.

Newton's Law of Conservation of Momentum is applied to understand collisions between vehicles.

Momentum is a vector quantity, and in head-on collisions, the momenta of two identical cars cancel each other out.

In collisions between cars of unequal mass, the lighter car experiences a greater change in speed and higher forces.

Kinetic energy, which depends on mass and velocity, is critical in car crashes and is proportional to the square of the speed.

The rapid transfer of kinetic energy is what causes injuries in car crashes.

Crashworthiness is a term used to describe the protection a car offers its occupants during a crash, involving many aspects of vehicle design.

The structure of a vehicle should remain intact during a crash to protect the occupants.

Crumple zones in vehicles are designed to absorb crash forces and protect the occupants.

Side-impact crashes require different safety considerations due to less crush space.

Physics teaches us the importance of seatbelts and the design of vehicles for crash safety.

The laws of physics dictate the dynamics of a motor vehicle crash, emphasizing the importance of safe driving choices.

Transcripts

Announcer: GENTLEMEN, START YOUR ENGINES!

Crew Member: GREEN, GREEN, GREEN, GREEN...

Griff Jones: 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.

HI, MY NAME IS GRIFF JONES. I TEACH HIGH SCHOOL PHYSICS.

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,

LIKE THIS STATE-OF-THE-ART HEAD PROTECTION SYSTEM.

WHAT'S EXCITING FOR ME IS THAT THIS IS A LABORATORY

OF PRACTICAL APPLICATIONS IN THE SUBJECT I TEACH.

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.

SO FOLLOW ME,

AND FOR THE NEXT FEW MINUTES I'LL TAKE YOU BEHIND THE SCENES

WHERE WE CAN EXPLORE THE BASIC SCIENCE

BEHIND VEHICLE CRASHES.

LET'S LEARN ABOUT CAR CRASHES AND PHYSICS.

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 100 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.

IT'S THE SAME PROPERTY THAT KEEPS THE CHINA ON THE TABLE

AS YOU PULL THE TABLECLOTH OUT FROM UNDER IT.

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 WE 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.

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 THE FORCE THAT OVERCOMES THE DUMMY'S INERTIA.

INERTIA IS ONE REASON THAT SEATBELTS ARE SO IMPORTANT.

INERTIA IS ONE REASON THAT YOU WANT TO BE TIED TO THE VEHICLE

DURING A CRASH.

IF YOU'RE WEARING YOUR SEATBELT,

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.

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=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 THE 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?

MOMENTUM IS INERTIA IN MOTION.

NEWTON DEFINED IT AS THE QUANTITY OF MOTION.

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=mv.

THAT'S MOMENTUM.

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 AND THROW EACH EGG WITH THE SAME VELOCITY.

THAT MEANS THEY HAVE THE SAME MOMENTUM.

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 MOMENTA.

AS A RESULT, IT WILL TAKE EQUAL IMPULSES

TO REDUCE THEIR MOMENTA TO ZERO.

ONE CAR WILL STOP BY PANIC BRAKING

AND THE OTHER BY NORMAL BREAKING.

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 A HIGHER DECELERATION.

A "g" IS A STANDARD UNIT OF ACCELERATION OR DECELERATION.

PEOPLE OFTEN REFER TO g's AS FORCES, BUT THEY'RE NOT.

FIGHTER PILOTS CAN FEEL AS MANY AS 9 g's

WHEN ACCELERATING DURING EXTREME MANEUVERS.

AND ASTRONAUTS HAVE FELT AS MANY AS 11.

PEOPLE IN SERIOUS CAR CRASHES EXPERIENCE EVEN HIGHER g's,

AND THIS CAN CAUSE INJURY.

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 g's.

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 g's.

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 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 CALL 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 GOING TO HAPPEN

WHEN I RELEASE THIS BALL AND IT COLLIDES WITH THE OTHERS?

LET'S TRY TWO.

IT'S AS IF SOMETHING ABOUT THE COLLISION

IS REMEMBERED OR SAVED.

NEWTON THEORIZED THAT THE TOTAL QUANTITY OF MOTION,

WHICH HE CALLED MOMENTUM, DOESN'T CHANGE.

IT'S CONSERVED.

THIS BECAME KNOWN AS THE LAW OF CONSERVATION OF MOMENTUM

AND IT'S ONE OF THE CORNERSTONE PRINCIPLES OF MODERN PHYSICS.

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.

INSIDE THE PASSENGER COMPARTMENT OF EACH CAR,

THE OCCUPANTS WOULD EXPERIENCE THE SAME DECELERATIONS

FROM 30 MILES PER HOUR TO ZERO.

THE DYNAMICS OF THIS CRASH WOULD BE THE SAME

AS A SINGLE VEHICLE CRASH INTO A RIGID BARRIER.

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.

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.

WEIGHT IS PRIMARILY AN ADVANTAGE IN A CRASH WITH ANOTHER VEHICLE.

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.

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'RE MOSTLY CONCERN WITH MOTION-RELATED ENERGY...

KINETIC ENERGY.

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.

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'RE EQUAL.

WHEN HAS THE PENDULUM REACHED ITS MAXIMUM KINETIC ENERGY?

HERE, AT THE BOTTOM OF ITS SWING.

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 THE KINETIC ENERGY

LOOKS LIKE THIS:

"KE," THAT'S KINETIC ENERGY,

EQUALS 1/2 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.

THE CONNECTION BETWEEN KINETIC ENERGY AND FORCE

IS THAT IN ORDER TO REDUCE THE CAR'S KINETIC ENERGY,

A DECELERATING FORCE MUST BE APPLIED OVER A DISTANCE.

THAT'S WORK.

TO SHED 4 TIMES AS MUCH KINETIC ENERGY

REQUIRES EITHER A DECELERATING FORCE

THAT'S 4 TIMES AS GREAT,

OR 4 TIMES AS MUCH CRUSH DISTANCE,

OR A COMBINATION OF THE TWO.

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.

Griff: THAT'S INCREDIBLE.

Brian O'Neill: ONE OF THE THINGS WE DO, WE PUT GREASE PAINT...

Griff: HE RUNS THE VEHICLE RESEARCH CENTER

AND IS ONE OF THE FOREMOST EXPERTS IN THE WORLD

ON VEHICLE SAFETY.

Brian: WE USE THE TERM "CRASHWORTHINESS"

TO DESCRIBE THE PROTECTION A CAR OFFERS ITS OCCUPANTS

DURING A CRASH.

NOW CRASHWORTHINESS IS A COMPLICATED CONCEPT

BECAUSE IT INVOLVES MANY ASPECTS OF VEHICLE DESIGN.

THE STRUCTURE, THE RESTRAINT SYSTEM,

IT ALL ADDS UP TO THIS SINGLE TERM WE USE, CRASHWORTHINESS.

WE USE THE STRIPPED-DOWN BODY

TO ILLUSTRATE THE CONCEPTS OF GOOD AND POOR STRUCTURAL DESIGNS

FOR MODERN CRASHWORTHINESS.

Griff: BRIAN, WHY IS IT IMPORTANT FOR THE VEHICLE'S STRUCTURE

TO PERFORM WELL IN A CRASH?

Brian: 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.

Griff: SO EVEN THOUGH ALL THIS METAL LOOKS THE SAME,

IT'S ACTUALLY DIFFERENT.

THIS, THE GREEN METAL'S INTENDED TO CRUMPLE,

TO GIVE IN THE COLLISION.

Brian: 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

OVER A GENTLER RATE.

Griff: RIGHT...KIND OF LIKE JUMPING OFF OF A STEP

AND KEEPING YOUR KNEES STRAIGHT AND LANDING ON THE FLOOR

VERSUS BENDING YOUR KNEES WHEN YOU LAND.

Brian: 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 MILES PER HOUR CRASH

AND AS YOU CAN SEE,

THE OCCUPANT COMPARTMENT IS 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.

Griff: SO EVEN IF THE RESTRAINT SYSTEMS DO FUNCTION PROPERLY--

THE AIRBAG, THE SEATBELTS--

THE PERSON IS STILL IN GREAT DANGER.

Brian: THIS PERSON IN THIS VEHICLE,

EVEN WITH A BELT SYSTEM AND AIRBAG,

IS AT SIGNIFICANT RISK OF INJURY

BECAUSE THE COMPARTMENT IS COLLAPSING.

Griff: 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 GOING TO BREAK THE CHINA.

Brian: WHEN THE SAFETY CAGE COLLAPSES,

YOU'RE GOING TO HAVE INJURIES TO THE OCCUPANTS.

SO THIS IS AN EXAMPLE OF POOR CRASHWORTHINESS.

BUT THIS VEHICLE WAS IN THE SAME CRASH...

40 MILES PER HOUR, OFFSET CRASH, AND YOU CAN SEE

THAT 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 SEATBELT AND PROTECTED BY THE AIRBAG,

COULD WALK AWAY FROM THE CRASH WITH NO INJURY.

Griff: 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.

Brian: THAT'S RIGHT.

THIS IS GOOD PERFORMANCE, GOOD CRASHWORTHINESS.

Griff: IN OUR SHIPPING BOX ANALOGY,

THIS IS AN EXAMPLE OF A STRONG BOX.

Brian: THAT'S RIGHT.

THE PEOPLE IN THIS BOX WILL BE PROTECTED.

Griff: BRIAN, OBVIOUSLY THIS CAR PERFORMED WELL,

BUT WHAT'S IN THE FUTURE FOR CRASHWORTHINESS?

Brian: THIS IS AN ILLUSTRATION OF HOW GOOD WE CAN DO

WITH FRONTAL CRASHWORTHINESS.

BUT FRONTAL CRASHES ARE ONLY PART OF THE PROBLEM.

WE OBVIOUSLY ALSO HAVE TO PAY ATTENTION TO OTHER CRASH MODES,

AND ONE OF THE MOST IMPORTANT IS THE SIDE-IMPACT CRASH.

NOW THIS WAS A VEHICLE THAT WAS IN A SEVERE SIDE-IMPACT CRASH.

THIS VEHICLE WAS GOING 20 MILES PER HOUR

SIDEWAYS INTO A POLE,

AND AS YOU CAN SEE, IN A SIDE CRASH

YOU DON'T HAVE ALL THE CRUSH SPACE YOU HAVE

IN A FRONTAL CRASH.

WE JUST HAVE THE WIDTH OF THE DOOR AND THE PADDING

AND, IN THIS CASE, WE HAVE AN AIRBAG ON THE INSIDE,

WHICH CREATES EVEN MORE SPACE.

WE INFLATE THE AIRBAG TO CREATE MORE CRUSH SPACE.

AND WE ALSO HAVE AN INFLATABLE AIRBAG

TO PROVIDE HEAD PROTECTION UP IN THIS REGION.

THIS DEPLOYS FROM THIS ROOF AREA HERE.

SO THE PHYSICS ARE THE SAME,

THE ENGINEERING CHALLENGES ARE GREATER.

Griff: I AM ALWAYS LOOKING FOR WAYS

TO RELATE THE PHYSICS THAT I TEACH

TO THE REAL WORLD THAT MY STUDENTS EXPERIENCE,

AND NOTHING IS MORE RELEVANT THAN TRAVELING IN AN AUTOMOBILE.

YOU PROBABLY DO IT EVERY DAY.

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.

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,

CAN MAKE A DIFFERENCE IN 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, 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.