Thursday, February 2, 2012

What is the role of a headrest in preventing injuries from whiplash, using Newton's 3 laws of motion?

I have researched Newton's 3 laws of motion, but I do not understand what they have to do with a headrest in a vehicle and preventing injuries from whiplash. Please help to explain this to me!What is the role of a headrest in preventing injuries from whiplash, using Newton's 3 laws of motion?Then you don't understand the physics.



Actually all three laws can be lumped into one physics characteristic... the change in momentum, dP, which is called impulse. Here's how.



Impulse is dP = m dV = F dt; where m is your head as you sit in the car seat, dV = V1 - V0 is the change in speed of your body as your car gets rear ended with a force F for an impact interval dt seconds. V0 is your head and body moving along at, say, 44 fps (30 mph), as you drive through a residential area. V1 is the new, faster speed V1 %26gt; V0 because your car gets pushed from behind with that force F.



Now here's the deal. Your head is up in the air while your body is snug against the seat pan and back when you get rear ended. So your head, when the crash occurs, wants to continue on at V0 (Newton's First) while your body is pushed faster to V1 (Newton's Second). What happens? Your head slams backward (actually it's your car slamming forward, but it's all realtive).



If there is nothing behind your head (i.e., no headrest), there is nothing to stop it from slamming backward except your neck muscles and bones. Whiplash as your bones are cracked and your muscles are strained while stopping your head from flying off your shoulders.



But with that headrest, your backward flying head is cushioned upon impact and brought to a stop so that the backward force is offset by the reaction force of the headrest. And there you have balanced equal but opposite forces on your head while you try to get the car under control and brought to a stop (Newton's Third).



And there you are... all three laws, using impulse as the central theme and the physics.

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