Understanding Car Crashes: It's Basic Physics

(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 b

asic 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 th

at 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 veh

icle 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 r

est 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 absorbi

ng 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 a

re 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 v

elocity. 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 spee

d, 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 mus

t 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 r

igid 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. T

his 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 pl

ace 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 moti

on, 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 compa

rtment 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 p

assenger 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. Sc

ientists 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 i

ts 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 cr

ash 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 occupan

ts 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 cr

ash 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 occ

upant 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 ove

r 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 f

igure 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 n

ext. - [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 o

ut 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 wheth

er 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)