I couldn't answer this on a quiz? Understanding force, impulse, and time; explain why an airbag is beneficial?

Question

My teacher went over this stuff so fast, I couldn't get all of the notes. The question is this:

Using the concepts of force, impulse and time (F*t), briefly explain why an airbag is a beneficial safety feature.

I need to understand this stuff for our test next week. There's no textbook for this source and it's a lecture hall course, so office hours for my professor are pretty filled up. Can someone explain this to me?

Thanks

Answer 1

In a crash, your body is thrown forward with momentum p = mv where m is the mass of your body and v is teh speed of the car at the moment of impact. Now if you stop over a very short time, your body experiences a force

F = dp/dt where dp = p - 0 = p and dt = time it takes to stop, So your momentum goes from some large number to zero in a very short time which implies the force experienced by your body is very big.

The air bag inflates very quickly and deflates slowly. It is the last part that is important becuase you want to reduce the force on your body that results from the momentum being reduced to zero. Since you can't change your momentum just prior to the crash, you have to somehow extend the time it takes to go from p to zero - remember F= dp/dt. So the air bag deploys, meets your body, and bcause it gives and releases gas slowly, it extends the time it takes to get rid of your momentum. Hence the force on your body is reduced to levels that are (hopefully) below those that can cause serious harm.

Answer 2

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Answer 3

I'm nt sre if ths is rite coz it was a wile ago wen i did it but i think that the force acting upon a body in a collision eg your head on the dash board in inversly propotional to the length of time of collision (impulse)
eg 100N in 1s = 100N hitting your head
100N in 10s = 10N htting your head

An airbag means a greater collision time
.... I think. u made need to put it in a better way but i think that is the basic idea.

I used to ind a website called physicsclassroom.com really usefull so u could try lookin there (maby under moentum section)

Hope this helps

Answer 4

An air bag’s job is to provide a cushion and inertia canceling device in case of a high speed crash.

A sensor in the bumper detects the crash and sends an electrical signal to fire a shotgun shell size initiator in the airbag. The bag quickly inflates in fractions of a second, just long enough to absorb the impact of the person then the bag deflates.

The idea is that when a car takes a sudden stop the passengers attempt to keep moving at the same speed. In the 1950s when steering wheel columns were rigid this caused drivers to die from a crushed rib cage, from an otherwise survivable crash. Collapsible steering wheel columns helped to reduce the number of fatalities, but the driver still took a lot of impact damage. This happened despite the use of seat belts, because the belt has to have some give to it. Otherwise it would cut deeply into the chest. As it is the seat belt tensioner works to absorb some of the impact energy, but only a little.

Ralf Nader, a safety expert, had been calling for the inclusion of air bags for over 20 years before Congress passed a law that required them. The car companies immediately claimed it as a “new” safety option.

When a crash happens now the air bag suddenly inflates in 0.05 seconds, at a speed of 220 mph, absorbing the person’s impact and then it deflates through vents in the bag. The airbag absorbs the impact completely before it has time to deflate.

So lets say you are going at 50 mph when you come to a sudden stop the seat belt tensionser absorb at most 5 mph, probably less so we can ignore them. That means the driver is moving at almost 50 mph across a distance of about a foot impacting the steering wheel. The force = ma so a 200 lb driver has 200 X 50 = 10,000 mph/lb force. Assuming he is actually accelerating to that speed. You should use the acceleration formula with 50 mph velocity to determine the actual acceleration at the point of impact, and then convert to feet/sec.

With the air bag and the seat belt the force is canceling out 220 mph of that 10,000 mph mph/lb force decreasing it to 9,780 mph/lb. Now when you convert that to foot pounds and only use the value of ACCELERATION not velocity then you will have smaller numbers and you can determine the exact deceleration force provided by the airbag. The vents on the side of the airbag prevent it from being a wall of force, because the bag absorbs the impact and this opens the vents so that force is used to expel the air inside of the bag.

According to Wikipedia: http://en.wikipedia.org/wiki/Airbags
“From the onset of the crash, the entire deployment and inflation process is faster than the blink of an eye (about 0.2 sec.). Airbags deploy in about 0.05 second. Because a vehicle changes speed so fast in a crash, air bags must inflate rapidly if they are to help reduce the risk of the occupant hitting the vehicle's interior.
Once an air bag deploys, deflation begins immediately as the gas escapes through vent(s) in the fabric (or, as it's sometimes called, the cushion). Deployment is frequently accompanied by the release of dust-like particles and gases in the vehicle's interior (called effluent). Most of this dust consists of cornstarch or talcum powder, which are used to lubricate the air bag during deployment. Newer designs produce effluent primarily consiting of harmless talcum powder/cornstarch and nitrogen gas (about 80% of the air we breathe is nitrogen).

An unrestrained or improperly restrained occupant can be seriously injured or killed by a deploying air bag. The National Highway Traffic Safety Administration (NHTSA) recommends drivers sit with at least 10 inches (254 mm) between the center of their breastbone and the center of the steering wheel.

The azide-containing pyrotechnic gas generators contain a substantial amount of the propellant. The driver-side airbag may contain a canister which is two inches in diameter, 1.5 inches long, and contains about 50 grams of sodium azide. The passenger side container is six inches long and contains 200 grams of sodium azide.[8] The incomplete combustion of the charge due to rapid cooling leads to production of carbon monoxide and nitrogen(II) oxide as reaction byproducts.[9]
The alternative propellants may incorporate eg. a combination of nitroguanidine, phase-stabilized ammonium nitrate (NH4NO3) or other nonmetallic oxidizer, and a nitrogen-rich fuel different than azide (eg. tetrazoles, triazoles, and their salts). The burn rate modifiers in the mixture may be an alkaline metal nitrate (NO3-) or nitrite (NO2-), dicyanamide or its salts, sodium borohydride (NaBH4), etc. The coolants and slag formers may be eg. clay, silica, alumina, glass, etc.[10] Other alternatives are eg. nitrocellulose based bipropellants (which have high gas yield but bad storage stability, and their oxygen balance requires secondary oxidation of the reaction products to avoid buildup of carbon monoxide), or high-oxygen nitrogen-free organic compounds with inorganic oxidizers (eg. di or tricarboxylic acids with chlorates (ClO3-) or perchlorates (HClO4) and eventually metallic oxides; the nitrogen-free formulation avoids formation of toxic nitrogen oxides).”