2019 Student Exploration: Crumple Zones Vocabulary: acceleration, airbag, collision avoidance system, crash test dummy, crumple zone, force, kinetic energy, Newton’s laws of motion, safety cell , seat belt, work, work-energy theorem Prior Knowledge Questions (Do these BEFORE using the Gizmo.) Two burglars run down an alley at night, trying to escape the cops. Jack is carrying a rigid metal safe. Jill is carrying an armful of antique quilts. In the pitch dark, they both collide headlong into a concrete wall. 1. Who do you think will be hurt more in the collision, and why? Jack who is carrying a metal safe would be more hurt because the impact of the safe against the concrete wall is more than an armful of quilts 2. During a car crash, what features of the car might act like either Jack’s safe or Jill’s quilts? During a car crash, the impact of the metal cage and steering wheel acts like Jack carrying a safe and Jill’s quilts act like the airbags of the car as they relieve the impact. Gizmo Warm-up When cars were first invented, the safety of passengers was not a great concern. As vehicles grew larger and faster, accidents became more deadly. Safety features went from being a rare luxury to a legal requirement. In the Crumple Zones Gizmo, you will design cars that will help a crash test dummy survive a collision. 1. To begin, do not make any changes to the DESIGN tab of the Gizmo. Select the CRASH TEST tab, and click Play ( ). After the crash, click Slo-mo replay. What happens?2019 The airbag and seat belt are missing. Due to this, the crash dummy goes forward hitting it’s neck on the wheel and the front of the car gets totaled. 2. Select the RESULTS tab to read about the results of the crash. Do you think a passenger would have survived this car crash? Explain. No because the impact of the car against the wall would lead to trauma/injuries the in head and torso. Activity A: Surviving a crash Get the Gizmo ready: • Click Reset ( ). • On the DESIGN tab, check that Sedan is selected. Introduction: Modern vehicles contain features designed to keep passengers safe in a crash. The crumple zone in the front of the car slows the car gradually and increases stopping time. The safety cell is a rigid cage that prevents passengers from being crushed. Inside, seat belts and airbags prevent the driver from hitting the windshield, steering wheel, or dashboard. Question: How does a crumple zone help protect a passenger? 1. Make a hypothesis: On the DESIGN tab, look at the parameters you can control. What settings do you think will make the safest car? Set up the Gizmo, and then fill in below. Crumple zone length: 120 cm Crumple zone rigidity: 200 kN Safety cell rigidity: 2000 kN Seat belt present? Yes If present, seat belt stiffness: 50 kN/m Air bag present? Yes If present, air bag rigidity: 0 kN 2. Test: On the CRASH TEST tab, check that the Car 1 speed is 16 m/s, or about 35 miles per hour (mph). Click Play. After the crash, select the RESULTS tab. A. By what percentage did the crumple zone deform? 77% Safety cell? 5% B. Did the dummy hit the steering wheel? No C. What was the maximum force on the dummy? 14.25 kN2019 D. How likely was the dummy to survive? 100% In this simulation, forces are measured in kilonewtons (kN). One kilonewton is equal to 1000 newtons, or the force of a 225-pound (102 kg) person standing on your chest. While many factors affect survival, only the maximum force and safety cell collapse are considered here. 3. Experiment: On the DESIGN tab, set the Crumple zone length to 100 cm and the Safety cell rigidity to 2000 kN. Set the Seat belt stiffness to 50 kN/m and turn off the Airbag. For each Crumple zone rigidity setting, run a 16 m/s crash test and enter the results below. Crumple zone rigidity Crumple zone deformation Dummy displacement Max. force on dummy Likelihood of survival 100 kN 100% 1.42 m 20.88 kN 87% 200 kN 93% 1.21 m 14.25 kN 100% 300 kN 62% 1.02 m 19.88 kN 89% 400 kN 47% 0.93 m 23.02 kN 82% (Activity A continued on nexActivity A (continued from previous page) 4. Evaluate: Look at the results of your experiment. What was the relationship between crumple zone rigidity, crumple zone deformation, and maximum force on the dummy? The relationship between all three factors depended on the rigidly of the crumple zones. Based off of that, determined the maximum force and deformaion 5. Infer: For a 1.00 m (100 cm) crumple zone, how much deformation do you think is needed in order to keep the passenger the safest? I believe a lot of deformation would be needed for example 0.50 meters. Explain your answer: As much space as possible in crumple zones keeps passengers the safest. This causes less force and therefore the survival rate high.2019 6. Explore: In the U.S., all cars are evaluated using a frontal 35 mph (~16 m/s) crash test. Using the variables on the DESIGN tab, try to design a car that produces the lowest possible force on the dummy and does not injure the dummy in a 16 m/s crash. If you like, record your data on the provided blank data tables on the next page. When you have finished, describe your car and your results below. Car parameters (CZ = crumple zone, SC = safety cell, SB = seat belt, AB = airbag) CZ length CZ rigidity SC rigidity SB stiffness AB rigidity 120 cm 200 kN 2000 KN 50 KN/m 0 KN Crash results CZ deform. SC deform. Dummy disp. Max. force Survival % 0.93 m 0.1 m 1.21m 14.25 kN 100% 7. Test: Click Reset. On the CRASH TEST tab, change the Car 1 speed to 22 m/s (about 50 mph). Run a crash test at this speed, then look at the summary data. A. What did you find? The survival rate changed to 0% and the front of the car was totaled. Not to mention, the maximum force on the dummy increased. B. What is a possible disadvantage of designing a car for only one crash speed? Disadvantages would be that the car would only be designed to be safe at a particular speed. This can be a major problem as vehicles travel at many various speeds. For example, if the car was designed for 50 kph, then that can be a major disadvantage as the car would become unsafe if the driver passed those speeds on a highway for example. This can cause many deaths from accidents. In order to combat this, car manufacturers would have to take into consideration that drivers go in a range of speeds including over the legal speed limit. This means they have to take speeds from 0kph up to almost 150kpm to ensure the drivers safety.2019 Activity B: Modern safety features Get the Gizmo ready: • Click Reset. • On the DESIGN tab, check that Sedan is selected. Introduction: The idea of a crumple zone was conceived by Béla Barényi in 1952 and first used in the 1959 Mercedes W111. Seat belts were also first widely used in the late 1950s. More recently, airbags and collision avoidance systems (CAS) were introduced. Question: How do modern safety features and body types help keep passengers safe? 1. Experiment: Set the Crumple zone length to 90 cm, the Crumple zone rigidity to 1000 kN, and the Safety cell rigidity to 1000 kN. Turn off the seat belt and the airbag. These settings represent a 1950’s car with no crumple zone, seat belt, or airbag. A. On the CRASH TEST tab, set the Car 1 speed to 16 m/s. Click Play. What happened to the dummy? There was no chance of survival in this case because the dummy flew into the steering wheel quite fast. B. Turn on the seat belt and set the Seat belt stiffness to 100 kN/s. Run another crash. What happened? The seatbelt was on this time, so the dummy didn’t hit the steering wheel causing no injury. C. Set the Crumple zone rigidity to 250 kN and run another test. What happened? The survival rate was at 95% and the dummy did not get injured. However, the front of the car was very smashed and damaged. D. How did the crumple zone and seat belt work together to keep the driver safe? The crumple zone and seatbelts kept the driver safe because there was less of an impact when it is bigger, and when seatbelts are used it prevents the dummy from flying forward due to inertia, which causes less of a risk of death.2019 2. Test: An airbag is designed to quickly inflate on impact, then deflate as the passenger hits the bag. Invented in the early 1970s, airbags did not become widespread until the 1990s. Using the crumple zone and safety cell settings above, experiment with the seat belt to find the lowest possible force on the dummy. Then, experiment with just the airbag. Finally, include both the seat belt and airbag. Report your findings below. Setup Seat belt stiffness Airbag rigidity Max. force Seat belt only 100 kN/m (No airbag) 16.55 kN Airbag only (No seatbelt) 30 kN 30 kN Seat belt and airbag 100 kN/m 30 kN 45.36 kN How did the seat belt and airbag work together to keep the driver safe? They both are designed for safety reasons to insure the passenger/drivers safety. They make sure the people in the car do not lean forward due to inertia. Instead it reduces that im (Activity B continued on next page) Activity B (continued from previous page) 3. Experiment: Another modern safety innovation is the collision avoidance system, or CAS. A CAS will apply the brakes when it senses an imminent collision. The brakes can slow the car by about 8.8 m/s for each second they are engaged. Click Reset. On the CRASH TEST tab, select Enable collision avoidance system (CAS). Drag the car to the far right of the track at the bottom of the Gizmo, then click Play. What were the results of this test? Due to the collision avoidance system being enabled, the car came to a full stop right before coming close to the wall. What this did was ensure the safety of the passenger with a survival rate of 100%, 4. Explore: Click Reset. In some cases, there will not be enough time for the car to completely stop before the crash occurs. However, the CAS can still be useful. On the CRASH TEST tab, set the Car 1 speed to 23 m/s (about 50 mph). Play a crash without the CAS, and then run another test with the CAS on. What did you find? The CAS slowed the car right before impact2019 5. Challenge: You work in the safety engineering department for a large auto manufacturer. Your job is to create the safest possible vehicle given each of the following design criteria. Using the Gizmo, try to create a vehicle that meets each description. • Design the safest possible SUV with a crumple zone length of 85 cm, no CAS, and a speed of 20 m/s (45 mph). (Note: Injuries such as broken legs are not allowed.) CZL CZR SCR SBS ABR Max. F Surv% 85 cm 350 kN 2550 kN 45 kN/m 10 kN 23.65 kN 80 % • Design the safest possible subcompact (no injuries) with a crumple zone length of 80 cm, no CAS, and a speed of 18 m/s (40 mph). CZL CZR SCR SBS ABR Max. F Surv% 80 cm 400 kN 2750 kN 35 kN/m 11 kN 22.74 kN 83% • Design a sedan with a crumple zone length of 110 cm that can give passengers a greater than 50% chance of surviving a 27 m/s (60 mph) crash and give passengers a greater than 80% chance of surviving a 16 m/s (36 mph) crash. (Note: Crumple zone length in both experiments is 110 cm.) Speed CZR SCR SBS ABR Max. F Surv% 27 m/s 800 kN 200 kN 700 kN 2000 kN 35 kN/m 50 kN/m 11 kN 0 kN 26.47 kN 74% 16 m/s 14.25 kN 100% Activity C: Force and acceleration Get the Gizmo ready: • Click Reset. • On the DESIGN tab, select the SUV. Introduction: To understand the physics of a car crash, it is helpful to consider Newton’s laws of motion: • First law: An object in motion will stay in motion unless acted on by an unbalanced force. • Second law: The acceleration (a) of an object is directly proportional to the net force (F) on the object and inversely proportional to its mass (m). In equation form: F = ma. • Third law: For every action there is an equal and opposite reaction. If object A exerts force F on object B, then object B exerts force –F on object A. Question: In a car crash, how are force, mass, acceleration, and velocity related?2019 1. Observe: On the DESIGN tab, check that SUV is chosen. Turn off the seat belt and airbag. On the CRASH TEST tab, set the Car 1 speed to 10 m/s and click Play. Observe the crash, then observe the dummy in slow motion by clicking the Slow-mo replay button. How does Newton’s first law explain the motion of the dummy? Newtons first law of inertia states that objects in motion tend to stay in motion and objects at rest tend to stay at rest. The phenomenon can be seen with the crash test dummy. As the dummy and the car are in motion, when the car suddenly stops, the dummy wants to stay in motion and that’s why it moves forward. 2. Select the TABLE tab. On the table, scroll down to where the Dummy v changes. The velocity change occurs when the dummy hits the steering wheel. A. What maximum force did the steering wheel exert on the dummy? -33 kN Notice that this force is negative. In this Gizmo, the positive direction is right to left. A negative force pushes the dummy from left to right, opposite its velocity. B. According to Newton’s third law, what force did the dummy exert on the steering wheel? + 33 kN 3. Calculate: On the DESIGN tab, set the Crumple zone length to 100 cm, the Crumple zone rigidity to 200 kN (200,000 N), and the Safety cell rigidity to 4000 kN. These settings will result in the crumple zone exerting about 200,000 N of force on the wall during the crash. A. If the crumple zone exerts +200,000 N of force on the wall, how much force will the wall exert on the crumple zone? -200 kN B. The SUV has a mass of 2,000 kg. According to Newton’s second law, what will be the acceleration of the SUV during the crash? -100 m/s^2 (Activity C continued on next page)2019 Activity C (continued from previous page) 4. Test: Select the TABLE tab and click Play. Look at the Car a column. Note where the SUV hits the wall and starts to slow down. What is the acceleration of the SUV? -95.1 m/s^2 You may notice that the acceleration of the car is not exactly what you predicted. The SUV’s acceleration depends on the rigidity of the crumple zone and the safety cell. If the safety cell were perfectly rigid, the acceleration would only depend on the rigidity of the crumple zone. 5. Predict: On the DESIGN tab, select the Subcompact. A subcompact has the engine in the back, so the entire front of the car is part of the crumple zone. (Engines do not compress easily, so having the engine in the front of the car reduces the length of the crumple zone.) Set the Crumple zone length to 80 cm, the Crumple zone rigidity to 190 kN, and the Safety cell rigidity to 4000 kN. Turn off the seat belt, turn on the airbag, and set the Airbag rigidity to 15 kN. A. Based on the crumple zone rigidity, approximately what force will the wall exert on the car during the crash? -190 kN B. Based on the mass, estimate the acceleration of the car: -200 m/s^2 C. Like the crumple zone, the airbag in this Gizmo exerts a constant force on the dummy. What force, in newtons, does the airbag exert? 15 kN D. The dummy has a mass of 50 kg (110 lb). When the dummy hits the airbag, what will be the acceleration of the dummy? -300 m/s^2 6. Test: On the CRASH TEST tab, set the Car 1 speed to 16 m/s. Click Play. Select the TABLE tab. What is the car’s acceleration and the dummy’s acceleration during the crash? Car acceleration during crash: -190.6 m/s^2 Dummy acceleration during crash: -300m/s^22019 How do these values compare to your predictions? My estimates for the car acceleration were close. However these values depend on the rigidity of the vehicle. 7. Summarize: How do Newton’s laws help to explain the acceleration of the car and dummy during a crash? Newton’s law helps us see that the acceleration of the car and the dummies force. Where the newton law tells us about how every action has a equal opposite reaction. The rigidity of the crumble zone will tell us the force that exerts on the car which will equal to the force applied to the dummy. Activity D: The work-energy theorem Get the Gizmo ready: • Click Reset. • On the DESIGN tab, select the SUV body type. Introduction: In designing a safe vehicle, the goal is to minimize the force on the passenger during a crash by maximizing the stopping time and distance. In this process, it is helpful to know the minimum force that is possible. One way to find this force is to consider how the kinetic energy of the dummy relates to the work done in stopping the dummy. Question: How can you determine the minimum possible force that acts on a passenger? 1. Calculate: The energy of a moving object is described by its kinetic energy, in which KE = ½ mv 2. Work is the product of force and distance: W = Fd. The work-energy theorem states that work changes the kinetic energy of an object: W = ΔKE. This means that the work required to stop an object is equal in magnitude to the kinetic energy of the object. A. Work is equal to Fd, and work is also equal to ΔKE. Combine these relationships to write an equation that relates force and distance to the change in kinetic energy. Then, rearrange this equation to solve for force. W=Fd , F=W/d KE=Fd , F=KE/d2019 B. If the goal of a safety system is to minimize the force, should the distance the dummy moves during the car crash be very large or very small? Explain your answer. Vey large because the first collision will make the dummy hit the airbag and will be pulled back by the seat belt. C. Suppose a car has a 1.00 meter crumple zone. The distance from the dummy to the steering wheel is 0.50 m (50 cm). After impact, if the crumple zone collapses completely, how far could the dummy go before hitting the steering wheel? Maximum distance dummy could be displaced: 0.49 m D. The dummy has a mass of 50 kg (110 lb). If the dummy is traveling 16 m/s at the time of the crash, what is the dummy’s initial kinetic energy? 6400 joules (J) E. ΔKE = KEfinal – KEinitial. If the dummy comes to a stop, what is ΔKE? -6.4 kj F. The work-energy theorem can be rewritten as F = ΔKE ÷ d. If the distance and kinetic energy change are known, you can solve for the force. Based on your answers to questions C and D, what constant force will stop the dummy? F = 13061.2244 N G. Convert the force to kilonewtons. F = 13.1 kN (Activity D continued on next page) Activity D (continued from previous page) 2. Test: In the Gizmo, set the Crumple zone length to 100 cm. On the CRASH TEST tab, set the Car 1 speed to 16 m/s. Click Play, wait for the end of the crash, then look at the Summary data on the RESULTS tab. A. What is the lowest possible maximum force on the dummy? 4.27 kN B. The RESULTS tab shows the magnitude of the force on the dummy. (Since magnitude does not deal with direction, it has no sign.) How does the magnitude of this force compare to the magnitude of the force you found in question 1G? The answer I got was higher than 1G. I found the lowest possible force on the gizmo than my calculation.2019 C. Suppose the dummy had a velocity of 24 m/s before the crash and the crumple zone had a length of 1.20 m. What is the lowest possible force on the dummy in this case? Show your work and check your answer with the Gizmo. KE = ½ mv 2 d= 1.2m + 0.5m = 1.7m KE = ½(50 kg)(24m/s^2)^2 = 14400J F=KE/d F= (14400J)(1.7m) = 8470N Minimum force on dummy: 8.47 kN 3. Calculate: In the simplified situation shown in the Gizmo, the exact design of the crumple zone is not considered. Instead, the crumple zone is modeled as a semirigid structure that exerts a constant force on the car as it collapses. (This model is similar to how real-world crumple zones act.) The more rigid the crumple zone, the greater this constant force will be. You can use the work-energy theorem to determine the ideal constant force the crumple zone exerts. On the DESIGN tab, select the SUV body type and set the Crumple zone length to 100 cm. Note the mass of the SUV is 2,000 kg. On the CRASH TEST tab, set the starting speed to 16 m/s. A. What is the kinetic energy of the SUV, in joules? 256,000 J B. How far can the crumple zone collapse, in meters? 0.26 m C. What force (in kN) must the crumple zone exert in order to stop the car as it collapses completely? 100,000 kN Check your answer in the Gizmo. 4. Think and discuss: No matter what you do, you will not be able to get the maximum force on the dummy exactly equal to the lowest possible maximum force because of the design of the airbags and seat belt. In the Gizmo, the seat belt is modeled as a spring, so the force of the belt increases as it is stretched. The airbag takes a few milliseconds to get into position. As a result, the dummy does not hit the airbag right away. How do these limitations affect how well the safety system can protect passengers in the car? What aspects of these systems can be improved? Write down your2019 thoughts in your notes, and then if possible, discuss these questions with your classmates and teacher. These limitations are built to stop the person in the car from getting injured or trauma in any way. In cars now, these systems are as improved as they could be in newer vehicles, however if you own an older one that’s not the case. In terms of what the future holds, there will probably be manufacturing of these things that are innovative, to make driving as safe as possible in terms of what factors the car can control. Nowadays, car companies focus on safety more than they did before. Simple systems like seat belts have the same mechanics and features as they did before. It needs to evolve. The problem with the seatbelt is that the material it is made up of is uncomfortable and can cause skin burns/irritation. To solve this, we can add padding to the seatbelt. Not to mention for passengers especially in the back where children sit, there should be a system in place where the car does not move until the children where their seatbelts. This can prevent many deaths and injuries. In speaking of kids, as the rear seats do not have airbags, the back of the front seats should have cushioning in case the child does not wear a seat belt. The reasoning for this is that the back of driver seats and front seats are made of hard plastic at the back. Airbags on the other hand are improved form before. Instead of having just two, many car manufactures add more than 8 to enhance safety. Newer cars have more sensors and AI built in. What this means is cars can now detect collisions more than ever. Which is why new cars much more safer than before.