I used to get the standard phone calls and emails that most physics professors get: the one from the guy who claims to have refuted Einstein's theory of relativity, the one from the guy who is positive that we never landed on the moon and it was all done on the soundstage, and the one from the guy who is really upset that Physical Review Letters won't publish his paper on the perpetual motion machine he's invented only because he doesn't have a Ph.D. and would I please submit the paper with my name on it because it would surely get published then. The last guy clearly hasn't seen my CV, or he'd know my success rate with PRL.
Jennifer wrote a great blog about mediagenicity a couple years ago that I recommend to everyone, and not just because she tagged me as 'a bit of a physics bombshell'. Before elucidating five rules for improving your mediagenicity quotient, she wrote:
"Mediagenics" are insightful, articulate, and likeably approachable, with a good sense of humor and innate ability to connect in any number of different ways with various audiences. It's making the connection that's so critical, because without that, communication is impossible.
If you're at all mediagenic - and in physics, the bar is pretty low -- the kinds of phone calls and emails you get change. Which is why I was standing out in the parking lot at school Friday night at 6:00 p.m. in 100 degree heat wearing a sweater over my sleeveless top because I dinged myself while moving things from one lab to the other and have a gigantic bruise on my upper arm. (Note added 6/28/09: You can see the results here.)
It all started Wednesday, when I got an email from Brandon Webb, who handles PR for the College of Natural Sciences and Mathematics at UT Dallas. He wanted to know if I could talk to a reporter who wanted to know whether a Styrofoam cup could break a windshield. (That's an advantage to being officially mediagenic - I don't get contacted directly now - they go through my 'people' a.k.a Brandon.)
C'mon - a Stryofoam cup break a windshield? Not likely. Then the reporter emailed me the photos. The windshield looks like a rock went through it just above the inspection sticker. For you non-Texans, that's a few inches from the bottom left side of the windshield.
Marilyn Mackey, the driver of the car with the broken windshield, was traveling about 65 mph down a highway. As a car passed her in the opposite direction, its driver threw a cup out his or her window. We know it was a cup of soda because, in addition to poor Marilyn getting wet, there is sticky dried cola-colored liquid all over the inside of the car. The police pulled part of the cup out of the windshield (which is how we know it came from Sonic) and found a shredded straw inside the car. (Thanks to the family for permission to post the photo!)
This called for a little research and one of Oullette's rules of mediagenicity is to take the call when it comes and appreciate that reporters work on hard deadlines, so it had to be quick. The Sonic website has a nutrition calculator, complete with masses. A small Coke is about 3/4 of a pound, while a Route 44 (the giant size) is about 2 lbs. A regulation baseball is about 1/3 of a pound, and the ginormous sode weighs about as much as a bag of sugar, just to put things in perspective. A gallon of water weighs 8.3 lbs. The weight of the cup is negligible compared to the soda. Yes, I did check: A Diet Coke weighs only very slightly less.
The two cars were coming at each other and we'll approximate that they were both going about 65 mph, which means that the speed of the cup relative to the car it hit was about 130 mph. (You never thought you'd need to use relative speed when you learned it in high school, did you?) If the driver of the oncoming car threw the cup instead of just releasing it, the cup would have an even greater speed when it hit the other car.
This is where the physicist side of me started to get fidgety. I get uncomfortable with numbers, because there's a lot we don't know. As you can see from the diagram, if we really wanted to do this right, we'd need to know how close the cars got to each other before the cup was thrown, how hard the cup was thrown, the angle the cup was thrown, the drag on the cup, how much of the drink was left in the cup...
Jennifer's second rule comes to mind - know your audience and tailor your message. Newspapers and television don't really want to know about all those details because they have to tell the story in a minute or two and caveats like that tend to take a lot of time to explain. And they DO want numbers. I explained to the reporters all the unknowns so they'd know why I was giving them a big a range of numbers. Take a conservative case: Let's assume that taking everything into account, the cup hit at a speed of 130 mph and came to a stop. (Again, an approximation because the cup went through the windshield and kept going.) Then the change in momentum (the difference between the momentum the cup had prior to hitting and the momentum the cup had after coming to a stop) would be:
where I've used that 130 mph=191 ft/s and made the distinction between weight (2.00 lb) and mass (weight divided by the acceleration due to gravity). The change in the momentum isn't the most important thing. The force is the change in momentum divided by the time it takes to change the momentum.
We have no idea how long the cup took to come to a stop, plus it hit the windshield, slowed down, and then part of it continued moving into the car. So let's just arbitrarily take a range from a tenth of a second to a half second. At a half second, the force would be about 24 pounds and at a tenth of a second, the force would be about 120 pounds. And that's the range I felt comfortable being quoted on in the newspaper article.
The reporter for Channel 8 asked me what the force actually meant. The best way to describe it would be that a scale placed on the windshield would register between 20 and 120 lbs when the cup hit. That quick calculation convinced me that it wasn't beyond the realm of possibility that a drink cup could actually break a windshield. If the cup were thrown, even a pretty bad arm could give it an additional 30-40 mph, so the force could have been much larger.
When we taped my interview for Channel 8, the reporter asked if it mattered how the cup was oriented when it hit. It does. Brandon - who is just a joy to work with - had pitched them the idea of taping the segment in front of a car using a Sonic cup as a prop, so I had the cup right there. This was a question that just came up, so I hadn't had a lot of time to think about it. That always makes me nervous because the last think you want is to be captured on tape saying something wrong.
It does make a difference. Compare what happens when a cup hits bottom first or side first, as I've tried to illustrate to the right. The bottom of the cup is really rigid, so there isn't going to be a lot of give. If it hits side first, the cup is going to give. If you've ever grabbed a flimsy drink cup and it squished and the lid came off, that's exactly what would happen. This is the exact same principle the SAFER barriers use for racetracks. Deforming the wall increases the time it takes for a car to come to a stop, and that decreases the force the driver feels. If the cup hit side first, it wouldn't create as much force as if it hit end first.
And, of course, I wasn't mentally or numerically agile enough to think to calculate the kinetic energy during the taping. A 2 lb cup of soda going 130 mph would have the same kinetic energy as a baseball thrown at 150 mph, or the same energy you get from exploding a half gram of TNT. I've spent the last year doing nothing but trying to get my lab up and running again. I apparently have lost the ability to do even simple math.
So I was only off by a factor of 2 or so.
(Total tangent: A 44 oz Coke contains 371 kilocalories of energy, which is equal to the kinetic energy of a passenger car going 86 mph.)
There are two important things you should take away from all of this. First, don't throw stuff out the window of your car. Ms. Mackey was wearing glasses and that's a really fortunate coincidence. The glasses were pitted from where pieces of windshield glass hit them. If she hadn't been wearing glasses, she might have had very serious injuries to her eyes. She could have lost control of the car and crashed and been killed. We are fortunate that this is a "what the...?" story and not a tragedy.
That leads to my second point. Did any of you ever consider a problem like this in any physics class you took? If not, why not? I spent about half a day answering questions about this situation from the media, from random people who emailed with follow up querstions. It's clearly something people are interested in. I mentioned this to one of my less free-thinking colleagues and was told that a problem like this is "simply too hard for beginning physics students". Balderdash. Doing calculations without friction when you like in a world where you can't escape friction is hard because nothing you calculate has any connection to real life. Figuring out whether a drink can go through a windshield and kill someone is complicated, but I didn't use anything you don't learn in first semester high school physics. I keep wanting to teach a course called 'Physics You'll Actually Use', but I haven't gotten anyone to approve such a title. Yet.
A couple of weeks back, a producer from Good Morning America left a voicemail on my phone wondering whether it would be possible for LeBron James to jump over a taxicab. We shot a segment and it ultimately never aired because the dumb ol' Cavs got themselves bumped out of the championship race. I'm counting on Shaq to get them there next year so that I can finally reveal the answer.