Impulse

[MedPR]

Can someone give me an example (or multiple examples) of each graph? For instance, which graph, if any, represents airbags during a car accident?

http://imgur.com/kmtSl

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[ljc]

Edit: nevermind, there it is.

Super quick, not great examples:

1. Car accelerating at a constant rate.
2. This is probably closest to airbags, due to the spring-like nature of airbags. As you get closer, the force increases. Then you bounce back, and the force decreases.
3. This would be like someone walking and kicking a soccer ball in front of them. Each bar is one kick. (10s is too long for one kick, obviously, but pretend it's 0.1s instead)

 

[milski]

Edit: nevermind, there it is.

Super quick, not great examples:

1. Car accelerating at a constant rate.
2. This is probably closest to airbags, due to the spring-like nature of airbags. As you get closer, the force increases. Then you bounce back, and the force decreases.
3. This would be like someone walking and kicking a soccer ball in front of them. Each bar is one kick. (10s is too long for one kick, obviously, but pretend it's 0.1s instead)

Only slight disagreement with 2: To bounce back, the force will have to change sign, here it's only decreasing. So it's hitting the airbag and then continuing through it as the air goes away.

 

[ljc]

Only slight disagreement with 2: To bounce back, the force will have to change sign, here it's only decreasing. So it's hitting the airbag and then continuing through it as the air goes away.

Ah yes. Whoops!

 

[MedPR]

Thanks guys So none of those graphs accurately represents airbags? Also, would airbags be more like graph 3 than graph 2 if they were graphs of the absolute value of the Force? Since at first you hit the airbag with high force, then at the point where you change direction (pushed back into your seatback) you have 0 force, then you once again experience increasing force (from the airbag)?

 

[ljc]

Thanks guys So none of those graphs accurately represents airbags? Also, would airbags be more like graph 3 than graph 2 if they were graphs of the absolute value of the Force? Since at first you hit the airbag with high force, then at the point where you change direction (pushed back into your seatback) you have 0 force, then you once again experience increasing force (from the airbag)?

If it were gradual, yes. But an airbag wouldn't put out two bursts of force like that. It would be more like a spring.

 

[milski]

Neither is great for an airbag - 10 s deployment is more or less useless. A realistic graph will have a relatively constant force pointed towards the back of the car which slows your motion forward. There are two reasons for a bounce back - the airbag deployed at a very low speed and you managed to bounce from it (I'm not sure how realistic that is) and your own muscles bringing you back. The idea of the airbag is for you to experience the deceleration from your speed to 0 in the longest possible time. What they are really trying to do is convert a collision wich looks like one of the green bars to something that looks like the blue one. The impulse stays the same but since you have more time for the force to act on your body, it can be a smaller forced, causing lower acceleration and less damage.

 

[MT Headed]

Only slight disagreement with 2: To bounce back, the force will have to change sign, here it's only decreasing. So it's hitting the airbag and then continuing through it as the air goes away.

When you smoosh into an airbag, the bag pushes you backwards, with increasing force. After the maximum compression, you start to bounce back but you are still compressing the airbag and it still pushes you towards your seat. At no point would the airbag start pulling you towards the steering wheel.

 

[milski]

When you smoosh into an airbag, the bag pushes you backwards, with increasing force. After the maximum compression, you start to bounce back but you are still compressing the airbag and it still pushes you towards your seat. At no point would the airbag start pulling you towards the steering wheel.

Yes, the force from the airbag is pointed backwards. The airbag is supposed to inflate before you hit it. I could not find any details on how fast the deflection process happens. If you were trying to minimize the force on the person, the ideal case would be to have it deflate as it is being compressed so that when the body comes at rest there is no force to bounce you back.

Allowing it to deflate too fast will decrease the force and eventually allow you to hit some other surface in the car since it would not stop you in the short distance it has. Making it deflate too slow will create a 'bounce back' effect which is an additional acceleration that decreases its efficiency. Where the balance is in the real product - I have no idea.

PS: Searching for images of inflating airbags must be done with some caution!

 

[MedPR]

Oh yea, EK asks which one of those graphs is an egg most likely to survive uncracked. Graph 1, correct?

 

[milski]

Oh yea, EK asks which one of those graphs is an egg most likely to survive uncracked. Graph 1, correct?

Yes, the force does the cracking. So the lowest the maximum force is - the better (for the egg).

 

[chiddler]

PS: Searching for images of inflating airbags must be done with some caution!

you have a very dirty mind. there's nothing remotely unusual lol

 

[milski]

you have a very dirty mind. there's nothing remotely unusual lol

No, I'm just very thorough.

airbag inflation video plus a few pages down.Not an abundance but it was the first thing that ended up in the middle of my monitor.

 

[SaintJude]

Yes, the force does the cracking. So the lowest the maximum force is - the better (for the egg).

Why isn't it graph 2? I thought the impulse (Force x time) is more important in this scenario. Increasing the time it takes for the same force to be applied would reduce the force of collision. In graph 2, it takes much longer for the force to reach 10 N.

In fact, this reminds me of airbags. Air bags are used in automobiles because they are able to minimize the effect of the force on an object involved in a collision. Air bags accomplish this extending the timerequired to stop the momentum of the driver and passenger.

 

[milski]

Why isn't it graph 2? I thought the impulse (Force x time) is more important in this scenario. Increasing the time it takes for the same force to be applied would reduce the force of collision. In graph 2, it takes much longer for the force to reach 10 N.

In fact, this reminds me of airbags. Air bags are used in automobiles because they are able to minimize the effect of the force on an object involved in a collision. Air bags accomplish this extending the timerequired to stop the momentum of the driver and passenger.

In graph two (red) the maximum force is 20 N vs 10 N in the blue one. The impulse in all cases is going to be the same m*v. Same story with the airbag - the impulse is always the same, you have to change the speed of your body from V to 0. The longer the time interval over which you do that change, the safer it is. As a matter of face, you add the same impulse to the system when you use your brakes to stop gently. Things start to get ugly only when you do the change in a very short period of time.

 

[SaintJude]

Oh, as usual, milski I see you are right. I keep on forgetting that the impulse on a F vs. t graph is the area under the graph. And the area is the same in graph 1 and graph 2. So, the impulse will be the same in both scenarios. I now see that the lowest maximum force will be best for the darn egg. Thanks milski!

 

[davcro]

When you smoosh into an airbag, the bag pushes you backwards, with increasing force. After the maximum compression, you start to bounce back but you are still compressing the airbag and it still pushes you towards your seat. At no point would the airbag start pulling you towards the steering wheel.

+1, the force from the airbag on the person doesn't change direction. The airbag pushes your head away from it during the entire impulse.