Last night I saw a classic conservation of momentum problem in person. It was about midnight, and I was on a service road beside west Houston’s Beltway 8 (avoiding the tolls) when I slowed down to stop at a red light. In my rear view mirror I saw the red and blue flash of emergency lights approaching, so with the room I had left I crept over a bit to the right to let them by. Whoosh! A car blew by my left side at high speed, swerving in front of me and speeding into the intersection heedless of the light. My neurons barely had time to start cooking up some surprise when a black pickup truck coming from the cross street entered the intersection from the right and neatly intersected the path of the speeding car with a tremendous and violent bang.
Conservation of momentum happened. The collision was not entirely inelastic nor was it entirely elastic. The truck struck the car very solidly on its right side behind the center of mass of the car, sending the car into a spin more or less along its original trajectory. The collision reduced the speed of the truck but didn’t change its trajectory much either, and the truck skidded to a halt. (A bonus friction problem!)
“Holy crap! This is a serious accident, I should pull over and try to help!”, I’d have thought to myself, if I had had time to form the mental impression into words. I didn’t have time, because within a second or two probably four or five police cars blazed past as well, surrounding the stopped and thoroughly totaled car.
“Holy crap! This wasn’t an accident, it’s a crime in progress! I might have to be ready to react to real danger,” I’d have thought to myself, if I had had time to form the mental impression into words. I didn’t have time, because the police jumped out with guns drawn and raced toward the car. I was in the process of pulling my car into the service station just to my right when the arrest happened, and though I didn’t see it clearly it looked like the driver was pulled from his car without appearing to be injured, arrested, and put into the back of one of the police cars.
Within a minute or two another dozen squad cars were on the scene, along with an ambulance and two firetrucks. The driver of the pickup truck seemed to be uninjured as well, and the ambulances and firetrucks seemed to be a precautionary measure. The police processed the scene and photographed everything, cleaned up the road with the help of the fire department, and later took a statement from me. They seemed pretty nonchalant about it, which I expect is because a witness isn’t really needed when there’s a bunch of dashboard cameras, two new-looking red light cameras on the intersection itself, and the driver of the truck as a more direct witness, aside from whatever other evidence they might have had for whatever caused the suspect to run in the first place.
It’s not really all that directly related to the practice of physics, but then again it’s not so often you see a classic Physics 101 problem instantiated in front of you in such dramatic fashion. In any event, I’m back in College Station now, tomorrow will have a quick July 4 holiday post, and then back to our regular schedule with a Sunday Function the next day. Enjoy your weekend, and remember that fireworks are physics too!