Car accidents
When cars get into accidents a lot of energy is transferred from car to car. This transfer causes injuries and ruins cars. To help explain the energy of a car accident, we must first explore some definitions and formulas.
Kinetic Energy-energy of an object that is moving at a constant speed. KE=(1/2)mv^2 Where ‘m’ is the mass of the object and ‘v’ is the velocity of the object.
Potential Energy-Stored energy due to the position of an object. Two types, gravitational refers to an object suspended at a certain hight, PE=mgy. Where ‘m’ is the mass of the object, ‘g’ is the acceleration due to gravity and ‘y’ is the height at which the object is located from a reference surface. Second type is elastic potential energy. This refers to the energy in a stored in a spring. PE=(1/2)Kx^2. Where ‘K’ is the spring constant and ‘x’ is the amount the spring is stretched of compressed.
Conservative Forces-The work done doesn’t depend on the path of the object. Work only depends on the initial and final positions of the object. This is done using the kinetic and potential energies of the object at the initial point and the final point.!
Nonconservative Forces-This will depend on the path of the object. Also, when friction or tension is involved.
Conservation of Energy-The total energy of an object at the beginning of observation must equal the total amount of energy at the end. KEinitial+PEinitial=KEfinal+PEfinal
Momentum-The product of an object’s mass and constant velocity. Given by p=mv. Where ‘p’ is momentum, ‘v’ object’s velocity and ‘m’ is the object’s mass.
Conservation of momentum-When the total amount of momentum before a collision equals the total amount of momentum after a collision. Pinitial=Pfinal or expanded for two objects m1vi1+m2vi2=m1v1f+m2v2f. Note that the direction of the objects must be taken into account, here ‘i’ stands for initial, ‘f’ for final and ‘1’ for object 1 and ‘2’ for object 2.
Elastic Collision-A collision in which the total kinetic energy and momentum before is equal to the total kinetic energy and momentum after. (Ideal)
Inelastic Collision-The kinetic energy before is not equal to the kinetic energy after. Some energy is lost due to heat. (Realistic)
During an accident, some energy is lost due as heat or friction. This means that these collisions are inelastic. To help protect passengers from getting hurt, cars are equipped with air bags and seat belts. When the car hits something, the car may stop, but the people inside won't. This is why you wear a seat belt when you're driving. The seat belt prevents you from being flung out of your seat and through the front window. The air bag is there to slow your body down, as well as increase the time of the impact. As the time of the impact increases, the force on your body will decrease.
Another component to car safety is the car's crumple zone. The crumple zone makes it so the car is able to crunch up, again increasing the time of the impact as well as decreasing the impulse on your body. A lower impulse will do a lot less damage to your body. Consider these three cars....
Kinetic Energy-energy of an object that is moving at a constant speed. KE=(1/2)mv^2 Where ‘m’ is the mass of the object and ‘v’ is the velocity of the object.
Potential Energy-Stored energy due to the position of an object. Two types, gravitational refers to an object suspended at a certain hight, PE=mgy. Where ‘m’ is the mass of the object, ‘g’ is the acceleration due to gravity and ‘y’ is the height at which the object is located from a reference surface. Second type is elastic potential energy. This refers to the energy in a stored in a spring. PE=(1/2)Kx^2. Where ‘K’ is the spring constant and ‘x’ is the amount the spring is stretched of compressed.
Conservative Forces-The work done doesn’t depend on the path of the object. Work only depends on the initial and final positions of the object. This is done using the kinetic and potential energies of the object at the initial point and the final point.!
Nonconservative Forces-This will depend on the path of the object. Also, when friction or tension is involved.
Conservation of Energy-The total energy of an object at the beginning of observation must equal the total amount of energy at the end. KEinitial+PEinitial=KEfinal+PEfinal
Momentum-The product of an object’s mass and constant velocity. Given by p=mv. Where ‘p’ is momentum, ‘v’ object’s velocity and ‘m’ is the object’s mass.
Conservation of momentum-When the total amount of momentum before a collision equals the total amount of momentum after a collision. Pinitial=Pfinal or expanded for two objects m1vi1+m2vi2=m1v1f+m2v2f. Note that the direction of the objects must be taken into account, here ‘i’ stands for initial, ‘f’ for final and ‘1’ for object 1 and ‘2’ for object 2.
Elastic Collision-A collision in which the total kinetic energy and momentum before is equal to the total kinetic energy and momentum after. (Ideal)
Inelastic Collision-The kinetic energy before is not equal to the kinetic energy after. Some energy is lost due to heat. (Realistic)
During an accident, some energy is lost due as heat or friction. This means that these collisions are inelastic. To help protect passengers from getting hurt, cars are equipped with air bags and seat belts. When the car hits something, the car may stop, but the people inside won't. This is why you wear a seat belt when you're driving. The seat belt prevents you from being flung out of your seat and through the front window. The air bag is there to slow your body down, as well as increase the time of the impact. As the time of the impact increases, the force on your body will decrease.
Another component to car safety is the car's crumple zone. The crumple zone makes it so the car is able to crunch up, again increasing the time of the impact as well as decreasing the impulse on your body. A lower impulse will do a lot less damage to your body. Consider these three cars....
The first image is of a Smart Car. Notice how small it is and how short the front of the car is. Both of these factors make this car unsafe. To help compensate for the small crumple zone, the Smart Car has a reinforced steel cage. While it's small size is economical, the car isn't practical. The next car down is of a Tesla Model S. Tesla stands by this car as one of the safest in the world. Notice the crumple zone in the front is much larger than that of the Smart Car. The third car is a Jeep Wrangle. Since this car is an SUV, it does have the biggest frontal crumple zone, but it also has a lot higher of a risk of rolling over. During a roll over accident you'd want a strong roof equipped with roll bars. Roll bars prevent the car's weight from crushing you if you land upside down.
Here is a video of a test crash for a Smart Car. Notice how the transfer of energy from the wall into the car makes the light weight car spring up in the air. This isn't something you'd want to see when that landing could be in another car's path. Also, at the end of the video there is a comparison with a normal sized sedan, again see how this car doesn't get violently tossed as much as the Smart car.
The next video is of a Tesla Model S. The Tesla has achieved the highest ratings on all of the crash tests preformed. The efficient crumple zone has a big impact on how passengers fair after an accident.
The next video is of a Tesla Model S. The Tesla has achieved the highest ratings on all of the crash tests preformed. The efficient crumple zone has a big impact on how passengers fair after an accident.
Let's talk more about an actual accident. I talked earlier about how the seat belt helps you survive an accident, let's go more in depth with that. Since the seat belt can stretch before it stops, it will increase the time of the impact. Increasing this time is key because Impulse=(Force)(Change in Time), and if the time is longer, the force will be smaller. But, there is also the energy lost due to friction and heat. If you were driving at 10 m/s and you hit a tree and say you bounce back at 5 m/s, how much energy was lost due to friction? Let's say your car has a mass of 1,400 kg.
Using conservation of energy, (remember the tree is still during the duration of the accident so v=0m/s)
(1/2)mv^2=(1/2)mv^2+Energy
(1/2)(1,400kg)(10m/s)^2=(1/2)(1,400kg)(5m/s)^2+E
Now, we can solve for the energy lost and we get E=52,500 Joules. This number seems huge, but it's a good thing, because without energy lost due to friction and heat, the energy of the impact would have been 70,000 Joules! This loss reduces the impact down to 17,500 Joules. To help put the Joule in perspective, one gram of TNT will release 4,184 Joules. So our energy lost is equivalent to 12.5 grams of TNT, but our actual accident is 4.2 grams of TNT!
To learn more about the key roll of a seat belt during an accident, click the link below.
Using conservation of energy, (remember the tree is still during the duration of the accident so v=0m/s)
(1/2)mv^2=(1/2)mv^2+Energy
(1/2)(1,400kg)(10m/s)^2=(1/2)(1,400kg)(5m/s)^2+E
Now, we can solve for the energy lost and we get E=52,500 Joules. This number seems huge, but it's a good thing, because without energy lost due to friction and heat, the energy of the impact would have been 70,000 Joules! This loss reduces the impact down to 17,500 Joules. To help put the Joule in perspective, one gram of TNT will release 4,184 Joules. So our energy lost is equivalent to 12.5 grams of TNT, but our actual accident is 4.2 grams of TNT!
To learn more about the key roll of a seat belt during an accident, click the link below.
The next link is a Prezi that I made for teachers to go through with a class about accidents, crumple zones, seat belts and air bags.
I've set up a form under the blog tab about car accidents, please give me your input!