Seat belts in cars are designed primarily to counteract the effects of:?

Question

* gravity
* acceleration
* inertia
* action/reaction

Answer 1

Inertia

"An object in motion will remain in motion until acted upon by an outside force"

Your body is moving at the same speed as the car. When the car stops, your body will keep going until it hits something (or until gravity eventually slows it down). As you will hit (and likely go through) the windshield long before gravity (which is also acting as an "outside force") slows you down, you need the seatbelt to act as the "outside force" needed to overcome inertia.

As far as the rest of your list:
The car overcame gravity when it started moving. The acceleration of the car caused you to get to a high enough speed to need the seatbelt's help. The action of hitting the seatbelt causes the reaction of you body overcoming inertia.

Answer 2

I'm going to suggest acceleration, contrary to the other answers. Why? Because F = ma; so when there is acceleration on a body (yours behind the steering wheel) as the car crashes into that street lamp post, you will be subjected to sudden deceleration (negative acceleration). That will put a lot of force on your body, which could really hurt (or worse) if you were to slam into the dashboard or windshield. And that's what the seat belts prevent; that sudden stop and resulting force on the dashboard and such.

Now, in response to those who answered "inertia." Inertia can be measured as momentum p = mv; where m is you sitting behind the wheel doing 120 mph = v in your little red Ferrari. But, what a minute, where's the force...where's the damage? There isn't any. Yet there is definitely inertia, you and your car are moving along the freeway at 120 mph. But there is no need for seat belts here. And, according to Newton's first law, that's the way things will stay unless a force (yielding an acceleration or deceleration) is applied.

But, as inertia can be measured as p = mv; a change in inertia over time is dp/dt = m dv/dt = ma = F. So, seat belts do not counteract "inertia," but they do counteract a change in inertia. And when there is a change in inertia, there is usually (not always) acceleration. [NB: There can also be a change in inertia if the mass changes over time, like in a rocketship that expends fuel mass. The complete change in inertia is dp/dt = dm/dt v + m dv/dt; but many times dm/dt = 0]

Answer 3

your body's inertial ability to accelerate through the windshield as a result of your moving body wanting to leave behind your stopped car...

Answer 4

inertia [matter in motion tends to stay in motion].

Answer 5

* tooth decay