The word “
collision” has a slightly different meaning in physics than in common parlance; in physics it simply means that two bodies exert force on each other for a relatively brief time. (“What counts as a brief time,” I hear you asking? It's not very rigorous, but generally it's brief compared to the whole time scale over which the system is observed.)
Within the definition of collision, there are also the concepts of elastic and inelastic collisions. Elastic collisions are where absolutely none of the kinetic energy of the objects involved is lost as another form of energy, such as sound, or heat, or mechanical energy as the objects deform. They generally don't happen on human scales, though there are many cases where it comes close: two billiard balls bouncing off each other is a pretty good approximation (the noise you hear as they collide is some of that kinetic energy being converted to sound).
Inelastic collisions, then are simply everything else, and what we deal with on a daily basis where two things collide with each other (in the colloquial sense) and some (or all) of the kinetic energy involved is converted to other forms of energy.
For example, I and my car were involved in a rather energetic inelastic collision this past Saturday up in Waimea. Before you ask, I'm
fine; almost remarkably so, in fact, as it could easily have gone much, much worse.
It all started in Waimea, where the Keck Observatory Headquarters are located. My soon-to-be adviser at Swinburne University in Melbourne was observing on Keck for a few nights (though remotely from Australia) and suggested I could come along to the remote observing room in Keck and watch and learn. I've never observed on Keck before, so I jumped at the chance to do so. The plan was to tag along two of the three nights he'd be observing (Saturday night & Sunday night) before heading back to Hilo.
 |
| Lovely Waimea. This is the view from where I stayed. Also the only picture I remembered to take this weekend. |
Things really kicked off with a bang—literally—as I was on my way back to the Keck HQ after arriving in Waimea, checking in, and eating dinner at the delightful Village Burger. I'd turned out onto the main road in Waimea (really just a stretch of the Māmalahoa Highway that encircles the island of Hawaiʻi), and was on my way back in the inner lane when I noticed a car coming the other way heading towards me.
 |
| My approximate route back from Village Burger to Keck. |
 |
| Me (cyan, heading north-east) happily returning to Keck after a delicious dinner. |
 |
| Me noticing another car heading straight towards me. |
It's interesting to observe how your brain tries to make sense of situations. In my case, I noticed a car crossing the center line heading towards me on a collision course a few seconds in the future and immediately assumed that a) he was following the rules of the road as a good driver, and that therefore b) he must be turning left and would either pull out in front of me (there was still time to do so) or would stop and wait for me to pass. I've only been to Waimea a few times and am not very familiar with it, or the fact that he was going backwards in a left-turn lane
for my side of the road might have tipped me off sooner.
Anyway, after a second or two (my recollection of the time involved is pretty hazy—it might've been even less, as we were both travelling perhaps ~30 mph [~50 km/h]) my brain realized that the probability of the hypothesis I'd formed of the other car stopping was very rapidly dropping to zero based on continued observation, causing me to swerve right at the last second. Thankfully I'd turned onto the road at a time when there were no other cars around so the road was pretty empty on my side; otherwise it would have been a multi-car pileup.
 |
| Me swerving and changing a ~60 mph Δv head-on collision into a violent sideswipe instead. |
I experienced the actual collision itself primarily as a side impact—both side airbags went off (but not the front ones), and the whole car got thrown to the side as the other car impacted in the driver's side rear door and wheel. The collision bled off a lot of the kinetic energy involved in the deformation of both cars, but the impact angle was also still low enough that both cars ricocheted off and kept going a bit farther.
I didn't have my life flash before my eyes or anything cool like that (despite the situation
clearly warranting it I think); at the actual point of impact the only emotion I can remember was a strong sense of resigned irritation that I
really didn't need this to be happening to me right now (especially when I was planning on selling my car in a few weeks to help finance my move to Australia), plus a nigh-instantaneous simultaneous unfolding of all the various consequences this was going to have on me for the next few weeks and how much I was already not looking forward to it.
 |
| The trajectories in this picture are mostly conjecture on my part based on where the two cars ended up. |
Very thankfully there weren't any other cars in the immediate vicinity, as the torque from the impact ended up spinning me around a bit and I ended up turned roughly sideways in the middle of the left-turn lane in the middle of the road, from where I had enough presence of mind left to limp across the street into the parking lot directly opposite, while the other car ended up on the far side of the road facing the wrong way.
To shorten a lengthy story (the details of which I'm slightly hazy on myself after this point for a while), miraculously neither I nor the other driver—a forty-something guy—were seriously injured, just shaken up a bit. The hospital and police station were just opposite the Keck building down the street, so they were on the scene fast, then there was a lot of talking and questions and paperwork being filled out, and a tow-truck being called, and the long and short of it is that I ended up grabbing the snacks out of my car and continuing to the Keck building on foot (less than a block away) where I went through with the night's observations as planned, though I did bail on the second night and get a ride back to Hilo from a good friend.
The other driver admitted to the paramedics that he'd been texting, which actually makes me the first person I know to get into an accident involving texting and driving. My car was unfortunately totaled, but thankfully Geico works fast and I got my insurance pay-out yesterday which is going to be quite helpful for what's turning out to be a…
more costly than I anticipated international move. (I'm borrowing my pastor's old extra car for now while I work things out.)
 |
| Couldn't get a good angle on it in the tow yard, but that's what it looked like. |
All in all, it's been a rather stressful week, but I at least have transportation for now and some much-needed funds in the bank. Now I'm free to get back to…oh, right. International moving preparations. Well, I'd better get busy. Don't text and drive, folks! A hui hou!
