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# Physics/newtons law

Question
QUESTION: Hello,

Using Newtons 3 laws of motion, how can i explain why the seatbelts should be used in a moving vehicle with logical arguments and scientific truths.

Thankyou

ANSWER: I'm in the hospital now (long story), so I'll make this brief.  Newton's laws state that you will be 1) moving in a straight line unless acted upon by an equal and opposite force.  If you happen to be strapped to a giant car while moving in a straight line then you will be subject to its rate of change of momentum, which will be much lower than if, say, you were to be ejected through a window and hit a really solid object.  Ouch!  Stitches.  They also say 2) That force = mass * acceleration.  Well, if you accelerate slowly, like the car with its crumple zones, you might accelerate less.  Especially if you're crashing into air bags.  And also 3)  That objects in contact exert equal and opposite forces on one another.  So seatbelts do bruise and injure you (just ask my mother) and should be held on really stiff springs to not catch so hard (some engineer just get working on that already), but essentially they still save you from going through glass, across pavement or into an oncoming vehicle, and dying in the bulk of cases.

---------- FOLLOW-UP ----------

QUESTION: Hope you get well soon.

Thanks for the answer, but what i dont get is how acceleration can relate to car accidents and enforcing to wear seat belts... If the car is accelerating and crash into something, how could it affect the driver other than the fact of inertia playing a role?

Thankyou

Ummmm...I think you just answered your own question.  Inertia would be the primary problem in a crash.  Seat belts are designed to decelerate the moving human body relative to whatever the car is crashing into.  I'm not sure how that's not somewhat obvious or how to make it more obvious, really.

Physics

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#### Dr. Stephen O. Nelson

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