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The technologies of safety features, such as roadside barriers, have been incorporated into the construction of highways and other major roadways.  A purpose of roadside barriers or guardrails is to prevent vehicles from swerving off the highway. If this is their only purpose, however, the most appropriate design would be the sturdiest-the concrete barrier.  Why, then, are other designs used?   What are the links between the laws of motion and the technology of roadside barriers?

Read The Design of Safety Devices Involves Varying «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi mathcolor=¨#1C82BA¨»F«/mi»«mrow mathcolor=¨#1C82BA¨»«mi»n«/mi»«mi»e«/mi»«mi»t«/mi»«/mrow»«/msub»«/math» and «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mo mathcolor=¨#1C82BA¨»§#8710;«/mo»«mi mathcolor=¨#1C82BA¨»t«/mi»«/math»
The answers to the above questions can be found on pages 463 to 465 of the textbook.  As you read these pages, note how the combination of the net force and the time interval are considered in the design of devices that protect people from injury.

Identify some of the safety devices incorporated into the design of automobiles that protect occupants during a crash.

Some of the safety devices that are incorporated into the design of automobiles include a padded dashboard, crumple zones built into the body of the vehicle, collapsible steering column, and airbags. 


Concisely explain the key physics principle behind the use of these devices.

Each safety device is designed to change the momentum of an object by applying an equivalent amount of impulse. This is described by the following rearranged version of Newton's second law: «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mover»«mi»F«/mi»«mo»§#8594;«/mo»«/mover»«mrow»«mi»n«/mi»«mi»e«/mi»«mi»t«/mi»«/mrow»«/msub»«mo»§#8710;«/mo»«mi»t«/mi»«mo»=«/mo»«mo»§#8710;«/mo»«mover»«mi»p«/mi»«mo»§#8594;«/mo»«/mover»«/math» The safety devices mentioned do this by increasing the time interval required to change the momentum so that the average net force required to stop the motion is minimized.


Many of the safety devices incorporated into the design of vehicles use the idea that the necessary impulse to change the momentum of an object can be applied in two ways.

    1. larger net force acting over a smaller time interval  
    2. smaller net force acting over a larger time interval

This concept has been applied to the design of roadside barriers.





When vehicles are traveling at the same velocity come to a stop, their impulse is the same regardless of how fast they stop, or how great the force was applied that caused them to stop. This is because their initial and final velocity is the same.

Many of the safety devices incorporated into the design of vehicles use the idea that the necessary impulse to change the momentum of an object can be applied with a smaller net force acting over a larger time interval or a larger net force acting over a smaller time interval.  This concept has been applied to the design of roadside barriers. 

So for a roadside barrier, if they were designed to increase the time it took to stop the vehicle, this means that during a collision with the barrier, the force acting on the vehicle as well as the passengers inside will be reduced.