Question

2

What happens in an automobile collision? Newton’s first law of motion states that an object in motion will stay in motion at the same speed and in the same direction unless it is acted upon by an unbalanced force. So for example, if a car is moving forward at 60 km/h, the passengers are as well. If this car collides with a solid wall or another vehicle, the force of the collision on the car is transferred to the passengers. Berenyi’s goal was to design a car that would reduce the force of a collision on passengers.

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Newton’s second law of motion states that the force of a collision  depends on the mass  and acceleration  of a vehicle  . Acceleration is a measure of how quickly the vehicle changes speed and is expressed as the change in speed over time. Therefore, as time increases for the same change in speed, acceleration decreases. And if acceleration decreases, so does the force of the collision on the passengers according to Newton’s second law.

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Crumple zones reduce the force of a collision on passengers by increasing the amount of time for the collision. This happens by allowing the front or rear of a car to crush during a collision. This crushing increases the time of the collision, and therefore decreases the force of the collision on passengers. Figure 1 illustrates how increasing the time of a collision exponentially reduces the force experienced by the occupants.



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The front and rear ends of a vehicle are designed to crumple in a controlled manner during a collision to give occupants additional time to safely decelerate in a crash. Crumple zones allow passengers to come to a slower, safer stop. Note that the controlled crush or crumple of the front- or rear-end, a positive safety feature, is totally different from the crush or collapse of the actual occupant compartment—which is to be avoided.

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The design of crumple zones differ from vehicle to vehicle—some deform more than others given the same amount of collision force. The stiffness of a vehicle is a measure that describes the relationship between the force of a collision and the amount the vehicle deforms.

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For example, to meet safety standards, a designer could modify the front structure of a vehicle to absorb more of the crash energy by making it less stiff (leading to more deformation). A National Highway Traffic Safety Administration (NHTSA) report, published in 1999, estimated the stiffness of a selection of passenger cars and light-truck vehicles, based on crash test results. Linear stiffness  was estimated using the following relationship:

where  is the mass of the vehicle,  is the velocity of the vehicle at impact, and  is the maximum crush of the vehicle. These estimates are shown in figure 2.



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Figure 2 shows that, in general, linear stiffness increases with vehicle mass. However, for any given weight there is a wide range of linear stiffness values. This results from the fact that for a given vehicle weight, vehicles display a substantial variation in the amount of crush, or front-end crumple, designed into the front structure. Light trucks and vans (LTVs) tend to crumple much less than passenger cars of the same weight and so generally, have higher stiffness values than cars.

Two-Part Question

Multiple Choice Question

The following question has two parts. First, answer Part A. Then, answer Part B.

Part A:

Which claim is made by the author of the texts?



Vehicles that have the same mass can have very different linear stiffness values.


Collision partners are the two vehicles involved in a collision.



Geometrically incompatible vehicles have a higher risk for passenger injuries because of a difference in the masses of the vehicles.



Linear stiffness of a vehicle depends on the amount of deformation and velocity during a collision, but is independent of the mass of the vehicle.

Multiple Choice Question

Part B:

Which sentence from the texts best supports this claim?



The stiffness of a vehicle is a measure that describes the relationship between the force of a collision and the amount the vehicle deforms.



This results from the fact that for a given vehicle weight, vehicles display a substantial variation in the amount of crush, or front-end crumple, designed into the front structure.



Collisions that involve two vehicles that are incompatible tend to have a higher potential for serious injury than do collisions that involve compatible vehicles.

D. 

In a frontal impact, a mismatch in the ride height of the colliding vehicles can lead to the misalignment of how the collision forces are applied to the vehicle structures.

Answer 1

Part A: Vehicles that have the same mass can have very different linear stiffness values.

Part B: "This results from the fact that for a given vehicle weight, vehicles display a substantial variation in the amount of crush, or front-end crumple, designed into the front structure."