Adventures in Applied Physics

My favorite part of my undergraduate major was studying kinematics. My favorite fundamental force is gravity. Unfortunately, there's not much more to discover in the field of kinematics, so I had to find something else to study in grad school.

But I am still an amateur kinematicist, and I perform experiments in the field nearly every day. I have a ten-mile commute to work every day (20 miles round trip), and it can get boring, so I amuse myself by attempting to leverage the force of gravity, friction, and my own ingenuity to use as little fuel as possible. I don't drive a Prius, but I strive for Prius-like mileage in my 2005 VW Beetle. It is a stick shift so I have complete control over what gear it's in. I'm also very familiar with the route, so I know where the hills and stoplights are.

I have two heuristics that I employ to minimize gas consumption: brake as little as possible, and coast as much as possible. These can be conflicting objectives, however, because when you're going down a hill towards a red light, you need to brake rather than take advantage of gravity, the natural accelerator. So there's a balance.

Also, braking is sometimes necessary, to avoid speeding and also to prevent stopping. Stopping should be avoided, because accelerating from 5 mph consumes 20% less fuel than accelerating from a full stop. Braking can be reduced by anticipating the behavior of the drivers around you. I stay way behind the car in front of me, so that I have the time to make fuel-efficient adjustments (e.g., coasting uphill, light braking) to my speed.

Because I am super geeky and I have my art merit badge (see the previous post), I decided to draw for you, my vast blog audience, a crude map of my daily journey. It's crude because first of all I freehanded it without looking at a map, and second of all I added my own contours that are probably completely inaccurate with respect to the true elevations, but they are my impression of the lay of the land. Finally, it is totally not to scale. (Part D of the map is more than half the journey, but it's boring so it doesn't deserve much space.)



The green circle represents home, and the red stop-sign represents work. The thick black line is the route I take, the thinner black lines are roads making up important intersections with stoplights, and the rainbow lines represent the elevation contours, with red being the highest elevation and descending through orange, yellow, green, and blue. So, work is at a lower elevation than home, meaning that my journey to work is mostly downhill, while my journey home is mostly uphill. But, you can see that there are some major hills in between: at A there is a valley, and at C there is a major hill. B is through the city, where it is mostly level but there are a number of stoplights. Somewhere along D, there is a guard shack, where you have to stop and show your badge in order to get on company property.

I can usually coast all the way from the intersection of my street with the main road (at the red contour) to the second intersection because that first stoplight is usually green. If I'm super lucky I can make it through the second stoplight too, although that usually doesn't happen. If I see it's red I just take the car out of gear and gently brake, in hopes that I won't have to stop if I arrive at the intersection later. (Of course I stop if I get there and the light is red.)

Part B is the city and if I go at just the right speed I can hit all the green lights. It is basically flat and pretty dull, except that there is this one intersection where the brains of people turning left into the Kroger shopping center atrophy or something. A statistically significant percentage of the times I am stopped at that light, the people in the first car in the line turning left into Kroger don't notice that they can go until it's almost too late. I don't know what it is: the radiation, the electromagnetic fields, or what, but this happens at least once a week.

It's not until I get to Part C of my jouney that things become kinematically interesting once again. There is a huge hill. I start at the bottom of the hill at 45 mph (the speed limit) and if nobody is behind me I take the car out of gear near the crest. At the top of the hill I am down to 25 mph, but once gravity (the natural accelerator) takes over, I begin to speed up, ending up at 45 mph at the bottom of the hill. Usually I can merge into traffic on the main road of part D without slowing down too much.

As I approach the gate, I take the car out of gear and coast. This probably drives the people behind me crazy, but relying on friction to decelerate saves a lot of fuel. I do usually have to brake, because I haven't figured out where exactly I should start coasting.

The 2005 VW Beetle gets 25 mpg city, 30 mpg highway, but I get better mileage than that. I estimate that I get about 33-35 mpg, which is over 20% more efficient than the standard.