Forces on wheels and fuel consumption in cars

Abstract

Motivated by real classroom discussions, we analyse the forces acting on moving vehicles, specifically friction forces on the wheels. In usual cars, with front-wheel drive, when the car accelerates these forces are in the forward direction in the front wheels, but they are in the opposite direction in the rear wheels. The situation may be intriguing for students, but it may also be helpful and stimulating to clarify the role of friction forces on rolling objects. In this article we also study thermodynamical aspects of an accelerating car, relating the distance traveled with the amount of fuel consumed. The fuel consumption is explicitly shown to be Galilean invariant and we identify the Gibbs free energy as the relevant quantity that enters in the thermodynamical description of the accelerating car. The more realistic case of the car's motion taking into account the dragging forces is also discussed.

Thermodynamical asymmetries in whirling, jumping and walking

“…A moving car, studied in [21], a falling cat, a long jump, etc. also provide examples whose thermodynamical analysis is parallel to the one described in this article.…”

Dissipation effects in mechanics and thermodynamics

“…The force on the tractive front wheel, powered by the engine but exerted by the ground, is represented in figure 2 for the car in its accelerating motion. As shown in [19] the force exerted on the rear non-tractive wheel points in the backward direction, but it is rather small and we neglect it here. Moreover, in the energetic discussion in [19] we considered the heat exchange with the air and the external work associated to the functioning engine, to conclude that the appropriate thermodynamical potential is the Gibbs function.…”

Thermodynamics in rotating systems—analysis of selected examples

“…A final remark on equation (26). For the steady state it can equivalently be expressed, after integration, as…”