I guess I should start a series titled "putting things on exploding air bags". Here is the next entry.
I shouldn't even blog about something like this. It just promotes something that you shouldn't do, actually I am upset. Why wouldn't you just use a TRIPOD! People, really, tripods are cheap. Oh, I get it. You spent all your money on this airbag. Ok, then just prop up the phone or camera on something. You are shaking this thing all over the place.
Ok, next question. Is this even real? Let's find out. Using Tacker video analysis, I get the following plot for the vertical position of the crazy airbag guy.
It isn't a perfect fit (mostly because it is a sucky video), but close enough for me to get an estimate for the maximum height of the guy. Let me conservatively say that his center of mass increased to a maximum height of 1.5 meters above the starting location. Here is the plan. Let me calculate the change in gravitational potential energy for this person moving up. Since the person didn't jump, this energy must come from stored energy in the airbag.
Here is the work-energy expression for the system including the person, the airbag and the Earth:
If I use a person mass of 70 kg, this would give a change in energy for the airbag with a value of -1029 Joules.
Now for another airbag video. Let me assume that the stored energy in the airbag is about the same. Here is my previous post for a – video showing a bowling ball on an airbag. Here is a plot of the vertical motion of the bowling ball (which was part of your homework from the previous post):
This looks nice. See what happens when you use a tripod? The acceleration seems to be correct and this gives a maximum height of 7.3 meters. The video lists the mass of the bowling ball (well, the weight) as 12 pounds (5.44 kg). This means that the decrease in stored energy for this airbag would be 389 Joules.
Ok, this energy seems to be significantly different than the stored energy in the person on the airbag video - like one third as much. Let me work backwards. Lets say that I know the airbag has 389 Joules stored in it. Also, let's say the guy is only 60 kg. How high would he go?
With these values, I get a height of 0.66 meters (2 feet). This has to be higher than two feet.
How can this be? Here are some options.
- The video is fake. This isn't my first guess. However, it is a pretty crappy quality video - this would make it easier to fake. I noticed that there seems to be two other guys with cameras. Where are their versions of this video?
- The tires. Perhaps when the airbag is in the two tires, more of the explosive energy is able to push the person. Also, the person is sitting on a board. With the bowling ball, there may have been more energy lost from the expanding airbag.
- Different airbags. Seriously. What do I know about airbags? Just about nothing. Maybe different cars have different airbags. I would assume they would all be similar, but maybe not.
At this point, if I had to pick, I would say option 2: the tires help more energy go to launching the person. Oh, don't try this silly stunt on your own. You could very, very easily hurt yourself or those around you. The acceleration from the launch alone could do serious damage - and then there is the collision with the ground. Bad idea.
Bonus Video
People always ask for bonus points in class, well here is your bonus. This is a video from Jeff Yap, Mike Belling, Tim Hughes, and Andy Harrison.
Using a camera from Tom Bird. Same idea as above, but with a slow motion video.
How does this video compare to the other one? Can you estimate the amount of energy stored in the airbag? Here is some extra info (from the description in youtube):
- Dummy mass = 50 kg
- There is a meter stick on the side (right side) of the air bag.
- Frame rate of 420 fps.
If you want, you could approach this in a simple way and just get the height of the dummy. Or maybe you want to do a video analysis, that would be cool too.
