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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 up to typically 10 km/s. When the engine cuts out, a probe will coast forever, following a curved path. The curve is due to the force of gravity from surrounding planets and the Sun acting on the probe. If there was no 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 the window of a moving car or waded through water, you've experienced it.) In physics, the definition of work is that work is done when a force moves a body through a distance. Kinetic energy (energy due to motion) is wasted doing work against drag. Since the energy was originally derived from fuel, in a vehicle driven by an internal combustion engine, it's a waste of that fuel. 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.