Built on Facts

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!


  1. #1 D. C. Sessions
    July 3, 2009

    Considering how little energy it takes to seriously screw up a bag of impure salt water, it doesn’t hurt to know what to do at the scene of a serious collision.

    Cops or not, you can’t always count on an ambulance arriving before you’ve cleared your car.

  2. #2 natural cynic
    July 4, 2009

    Are you anxiously waiting for your incident to make Most Shocking Car Chases?

  3. #3 lurker
    July 4, 2009

    I’m glad everyone is okay, especially you and the driver of the truck. There are too many awful incidents where chases like that end in someone accidentally getting hurt or killed.

  4. #4 Jim H
    July 4, 2009

    Forgot to mention the propulsion of the car via internal combustion was pure physics, too.

    And in all fairness, you should give chemistry proper credit for fireworks. Nice post.

  5. #5 physicist in transit
    July 4, 2009

    Another good conservation of momentum story from my past. Sitting in a stopped SEPTA (Philadelphia) bus that just discharged a passenger, a drunk plowed into the back of the bus at ~40mph. We heard it, and slightly felt the bus move. But the far larger inertia of the bus made the crash a non-event for its occupants. Not so good for the car. And once the police arrived, not so good for the car’s driver who was suffering more from drink than deceleration.

  6. #6 Carl Brannen
    July 4, 2009

    The nonchalance of the police in these situations is something I’ve noticed. I went to a Seattle area gun range to settle the question of who was the best shot with my buddy (I was born in Texas so you know the answer). It was fairly late at night. When we got done and left the range, the woman who’d been shooting next to us deliberately killed herself. Apparently she’d been waiting for us to leave so as to have some privacy, or perhaps to avoid horrifying us. Since I was the last person to see her alive I stayed to give a statement to the police. They told me that they had the whole thing on tape and didn’t need anything. The police acted like this sort of thing went on all the time.

    By the way, as a result of this, I made a resolve to always chat up strangers. A person told me that once, when they’d been planning to kill themselves in a 1-car “accident” (so that relatives could think that it was not suicide), he stopped at a rest stop and while he was going over to get a drink of water or take a last dump a young boy stopped and asked him about his beautiful car. That was enough to snap him out of the mood.

  7. #7 Uncle Al
    July 4, 2009

    Denying the State its projected income is a summary capital offense. You’ve been warned by example. Care to try for twosies?

    If you think bearing witness alters the course of a revenue-aggrandizing trial, sit in on traffic court or IRS court. No jury, no recording allowed. First the verdict, then the trial. Freedom is compliance. Red light cameras are private enterprise sharing revenues. Their timing is rigged to support business plan income spread sheet projections. That is what “remotely programmable for updates” means. “We’re a little short this month. You ran a red light and here is the photo to prove it.”

  8. #8 Art
    July 4, 2009

    You might want to give the police department a call and explain your profession and desire to teach and request that you get copies of any tapes, photographs and reports. There is likely a public relations officer for the department who might be won over by noting your trying to educate the kids and this will reinforce the dangers of hazardous driving and why you need to wear seat belts.

    Usually, from my limited experience, the police will have a scaled diagram of the intersection, measurements of any skid marks, some estimates of the speeds involved and a diagram of where the vehicles ended up. From all this you should be able to make a nice audiovisual presentation of a physics problem.

    Given that any recordings have a frame rate and you know, or can get firm measurements of the intersection by going out there with a tape measure you can check the police estimates of speed against your own calculations. Another aspect to explore. The make and model of vehicle will be on any report and the designed GVW of the vehicles available online. You could even include the weight of the drivers. The guy arrested will have his weight estimated on the form. The police who were at the scene should be able to give you an estimate wight of pickup driver.

    You can get as deep as you like. The automakers could give you figures on the vehicles crumple zones and how much energy they are designed to absorb. You, or your students, could track down the wrecked vehicles and measure the deformation and calculate how changes in impact location and speed might change the outcome. The sky is the limit.

    As you point out at the very least you have a gripping problem in elementary physics that should keep student’s attention for a time. In theory you could take this one event and make it into subject matter for a day or two and, if yo get into the details, perhaps create an entire class around it. Perhaps the general subject might become a multidisciplinary mix of physics, mechanical engineering and traffic engineering. Wouldn’t that be a sweet can of worms.

  9. #9 Eric Lund
    July 4, 2009

    Nice suggestion, Art, but Matt may want to wait until the case is disposed of (whether by trial or by plea). IANAL, but the police may have good reasons for not wanting to discuss a pending case–there is at least the theoretical risk of giving the defendant’s lawyer something, anything, to hang a case on. It worked for O. J. Simpson in the 1990s: his legal team was able to show that the LAPD engaged in an attempted (and probably unnecessary) frame-up, and that was enough to introduce reasonable doubt (which is sufficient to find for the defendant in a criminal case) in the minds of the jurors. Having an overt racist like Mark Furman leading the police case against a black man probably didn’t help the prosecution, either.

  10. #10 CCPhysicist
    July 5, 2009

    The driver of that truck had his life saved by a fraction of a second. Just a tick later and the car hits driver side (based on your description) and you might have been a witness to vehicular homicide or even capital first degree murder depending on details of the law in Texas and what was behind the pursuit.

    Don’t discount your importance as a witness. The truck driver is a victim and thus less important than you as an independent observer.

    Some of the interesting physics is that the spin probably helped dissipate energy more gently than a square impact if the speeds were as high as you imply.

New comments have been disabled.