Driving Slower to Reduce Fuel Consumption

Created by ChatGPT

In theory, according to Newton's first law, a body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a force. So if you kicked a ball in space and there were no planets or stars to influence the ball, it would go on forever.

That's how deep-space probes work: during launch, engines fire to create thrust and increase velocity typically up to 10 km/s or more. When the engine cuts out, a probe will coast indefinitely, following a curved path due to the gravitational pull of planets and the Sun. Without gravity, the trajectory would be a straight line.

In the real world however, forces tend to slow things down. In the case of a vehicle travelling on a road, after a driver takes their foot off the accelerator pedal, velocity drops. That's because of friction at the wheels, but predominantly due to resistance from the fluid the vehicle is passing through, i.e. the air. That viscous friction force is known as drag. (If you’ve ever put your hand out of a moving car window or waded through water, you’ve felt it.)

In physics, the definition of work is that work is done when a force moves a body through a distance. Energy is defined as the ability to do work, and kinetic energy (energy due to motion) is therefore wasted doing work against drag. In a vehicle powered by an internal combustion engine, this energy originates from fuel (or stored chemical energy in the case of an electric vehicle). Therefore, this represents wasted fuel or electricity used to charge the vehicle. Vehicles are normally streamlined to reduce drag and improve fuel efficiency.

However, drag can also be reduced and fuel efficiency improved simply by driving slower. It turns out that drag is proportional to the square of the velocity of a vehicle. So a vehicle travelling at 100 km/h has double the velocity of a vehicle travelling at 50 km/h. (In physics, we normally use velocity rather than speed. Velocity is speed in a given direction and is a vector quantity.) However, drag experienced by the faster vehicle is four times that experienced by the vehicle travelling at the slower velocity, due to the square factor. Since work done against drag equals force multiplied by distance and the distance is the same in both cases, energy wasted overcoming drag is four times greater for the vehicle travelling at 100 km/h.