# Homework Help: Science behind car safety features (broad, open-ended question, sorry)

1. Sep 11, 2010

### deman

Hi all, I'm new here and new to physics all together. I am taking a general science class in college (real basic stuff) and have only been focusing on physics for 1 term so im pretty ignorant on the topic.

We have an in class assessment dealing with cars and their safety features: seatbelts(including inertia reels), airbags, crumplezones and brakes; dealing with the science behind their function. I need to refer to Newtonâ€™s Laws of motion, The Equations of Motion, energy transfers and momentum where appropriate.

I am aware that the science in car crashes is all about reducing the stopping time, both of the car and occupant and that safety features need to take the occupants kinetic energy (or momentum??) and dissipate it over time to reduce injury.

Other than the conservation of energy and Newton's first law, what other equations theories should i relate to these safety features and their function?

Any input or just a general push in the right direction would be greatly appreciated.

2. Sep 12, 2010

### lewando

I'm not sure about the "reducing the stopping time" part of this statement. What kills people in a crash is there is not enough stopping time. Let's say you jump from a building and hit the street. This would be an example of zero "stopping time". If you land on a fire department air matress instead, your "stopping time" would be longer. In the first case, you are subject to intense deceleration. In the second, a more gentle deceleration.

For cars, think crumple zones and air bags.

3. Sep 12, 2010

### deman

Yes, my bad. I actually meant increase both stopping distance and stopping time. I've done some research and this is what i have so far, I don't know how much is right.

Seatbelts
- A seatbelts job is to act against Newtons 1st law (inertia) and prevent the occupant from hitting the steering column or windshield (in which the stopping time would be 0).

- Seatbelts that stretch upon collision offer a longer stopping distance, applying less force on the occupant.

relevant equations: V = d/t, p = mv, Work = force x distance

Airbags
- Airbags increase the time during which the occupants momentum decreases (stopping time)

- The large surface area of the airbag helps dissipate energy equally over the full surface area of body resulting in less injuries.

relevant equations: V= d/t, pascals principle.

Brakes
- Work with Newtons second law by decreasing acceleration and therefore decreasing the force behind the vehicle.

- Brakes turn kinetic energy into heat energy through the friction between the brake pads and wheel rotor and the friction between the tyres and road, thus decelerating the vehicle.

relevant equations: F = ma.

Still working on Crumple zones but i imagine they work in the same manner as airbags.

Please feel free to point out any incorrect statements and to add to any equations/theories. I'm sure im missing something.

Last edited: Sep 12, 2010
4. Sep 12, 2010

### Staff: Mentor

I think you are doing great.

You may try to analyze most of the cases in terms of safe way of dissipating kinetic energy.