Diandra Leslie-Pelecky, a physicist who’s got a good rep with her local news media, describes the process she went through to calculate the sound-bites explaining how a styrofoam soda-cup thrown from an oncoming car managed to smash the windscreen of the car it hit.
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. Sonic_CupShape
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. (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.)