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Car Crash Design Lab This activity is designed to utilize your prior knowledge of physics and physical systems in order select critical safety features of a car. In this lab you are asked to select components of a vehicle that will enable a human passenger to survive a head on collision while travelling 45 mph. In order to make educated selections for your car components, you must first become familiar with the manner in which they interact. Problem Statement: You have been asked to select the final three design components that will complete the construction of a car, but you must also make these choices without going over your allocated budget. The three choices you have to make are as follows: Bumper: The bumper of the car is the first component to come into play during a head on collision. You will be asked to select a bumper based on a given price and quality factor which will be explained later during this introduction. Hood: The next component to consider is the hood of the car. The hood acts as the second feature experiencing force from the crash. You will be asked to select a bumper based on a given price and quality factor which will be explained later during this introduction. Airbag: The air bag is the final component that can help to reduce the amount of force experienced by the driver. Again, you will be asked to select a bumper based on a given price and quality factor which will be explained later during this introduction. How they interact: In order to understand how the different components work together we must first understand how they work individually. For this lab, each component will be treated as either a spring or as a system of springs. The three pieces together form a system of springs that absorb impact from the crash and help reduce the amount of force experienced by the driver. There are three different ways in which springs can be represented. They can be either: a) A single spring b) Springs in series c) Springs in parallel As you have previously learned, a spring is associated with a spring constant k, which in essence determines the strength of the spring. In this lab you will have to determine the overall spring constant of the system by using the appropriate formulas as well as the values associated with your component selections. For each component of the car, you will be making a decision that affects the overall strength of the system. The above table shows you how you are to calculate the overall spring constant for a given system of springs. The following section of this introduction will show you what factors to base your component selection process from and how to calculate each pieces contribution to the system. Bumper: Bumper Spring Constant k B– Number of Springs in Bumper NB The bumper can be visualized as a component directly corresponding to the “In Parallel” spring system. The factor you will be choosing is the number of internal springs contained within the bumper. Regardless of how many springs you choose for your system to have, the spring constant for each spring remains the same. This makes the overall spring constant for the bumper: Beq = NB * kB Hood: Hood Spring Constant kH – Number of Crush Points in Hood NH The hood can be visualized as a series of springs due to the nature of the hood’s “crumple zones.” These crumple zones, or crush points, act as individual springs along the length of the hood. In order to help understand this effect, consider the following images: You will be choosing the number of crumple zones contained within the hood of the car. Regardless of how many springs you choose for your system to have, the spring constant for each spring remains the same. Each zone acts as its own spring. Using this information you can calculate the overall spring constant for the hood by: 1/Heq = Hn / kH Airbag: Airbag Spring Constant kA = Aeq The airbag can be thought of as a simple 1 spring system that attempts to absorb any leftover force from the crash and reduce the impact to the driver. The factor you will choose for the airbag is the material it is made from which will affect the spring constant for the component. You are essentially choosing the ka factor for your airbag. Overall system: Overall Spring Constant - keq Once you have made your selections you can now determine the overall spring constant for the system. Here is the breakdown we have looked at so far. 1) The bumper is a spring system of springs in parallel 2) The hood is a system of springs in series 3) The airbag is a single spring Taking these three components and viewing the overall system as one large series of springs, you can refer once again to the formula for springs in series and calculate the total by: Overall Spring constant for system: 1/keq = 1/ Beq + 1/ Heq + 1/ Aeq Using this information: Now that you know how to calculate the overall spring constant for your system, you will need to know how to use this information to determine the level of safety in the vehicle you design. Designing you car: There are two critical elements to consider when thinking about the impact your car will have during its crash: 1) The car’s mass 1360 kg 2) The velocity it is travelling 45 mph Scientific research has shown that the amount of force a person experiences during a crash relates in the following fashion: Magnitude of Deceleration Sustained Injuries Acceptable Outcome (measured in G’s) 0 ≤ F < 12 Little to no injury Yes 12 ≤ F < 20 Minor Yes 20 ≤ F < 35 Serious Injuries No 35 ≤ F < 50 Serious Injuries with No possibility of Death The potential energy residing in each spring prior to the crash is the key factor in limiting the amount of force transferred to the driver. The potential energy present in each spring in dependent on both the spring constant and its maximum displacement from equilibrium. Process: In order to ensure survival of the driver, it is necessary that they sustain less than 40 G’s of force during the crash. In order to make this occur, the entire spring system must be capable of mitigating much of the initial impact. The manufacturers of the car components provide you with the corresponding spring constant for each piece. This information will be available within the design simulation program. You are now ready to refer to the lab worksheet and begin designing your vehicle.
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