I spend large portions of my life driving up and down Interstate 5 in order to visit my husband, who lives in Seattle. As a driver and a graduate student in physics, I am struck that my fellow drivers are often quite ignorant about basic physics concepts such as velocity, deceleration (how fast something is slowing down), sliding and momentum.
I play a game with myself: Is the person in that car up there weaving back and forth, going way too fast and tailgating because they are a) asleep at the wheel, b) stoned, c) a jerk or d) not too bright? There are, of course, other options. The driver could be not paying attention, distracted or ignorant about the physics of driving.
I think a little attention to physics would improve the driving of most people. So, here are two examples of scary things I have observed.
* Tailgating: When someone follows so closely behind another car that if the other car brakes suddenly, the reaction time of the following driver is not swift enough to brake completely before running into the lead car. Our reaction time is basically constant, no matter how fast we are traveling.
Recently I was driving down 18th Avenue with wet roads and heavy traffic and a tailgater right behind me. A stoplight turned yellow, I braked, and the car behind me braked so hard that I couldn’t see the headlights in my rearview mirror except while the car was still bouncing up and down from the sudden stop. If you run into a heavy, stopped vehicle your car will stop before you do. Your whole body will continue moving at 30 mph until parts of it are stopped individually: your torso by your seatbelt and your head by your neck. This is why neck injuries occur in this situation. You have momentum, and your car has momentum.
Momentum is mass times velocity; since you mass about 10 percent of your car’s mass, your car has the greater momentum. Things with momentum which are suddenly stopped have to transfer their momentum. For inelastic collisions, some can be lost as heat and crumpling of the car body itself. Or, the momentum can be transferred to people in the car (this is how people get thrown through the windshield).
* Sliding without rolling: It is winter and has recently rained. Overnight the temperature has dropped and the bridges (being in contact with more air than ground, and the air temperature being lower than the ground temperature) are a little icy. I’m near Centralia, Wash., and there is a little slowdown half a mile ahead. The car ahead of me hits an overpass going about 70 mph and while on the bridge tries to brake in anticipation of the slowdown. The car begins to turn sideways while not slowing down. The driver panics and hits the brakes hard and turns the steering wheel too much. He gets off the ice and because he has overcorrected with the steering wheel manages to drive off the road and off the shoulder and only barely succeeds in driving back onto the shoulder without sliding down the bank on the side of the road.
Moral of the story? Don’t overcorrect your steering and don’t hit the brakes hard while in puddles or on ice.
Drive safe, folks!