Another cause of energy loss during motion is air resistance. It arises from the continuous transfer of kinetic energy from the moving object to the air molecules in its path.
The mass $m_\mathrm{L}$ of the displaced air can be expressed as the product $\varrho_\mathrm{L} V$, where $\varrho_\mathrm{L}$ is the density of the air, and $V$ is the displaced volume. This volume can be approximated as the product of the object’s cross-sectional area $A$ and the distance traveled $s$. Substituting into the kinetic energy formula yields:
$$
E_\mathrm{L} = \frac{1}{2}\varrho_\mathrm{L} A s v^2
$$
It must be noted that the object’s actual surface area may not be equal to the effective area facing the airflow. Therefore, the equation must be corrected with a factor $c_\mathrm{W}$, known as the drag coefficient, which has no unit and can only be determined experimentally or via simulations.
Dividing the energy expression by the distance $s$, we obtain a formula for the air resistance force:
$$
F_\mathrm{LW} = \frac{1}{2} c_\mathrm{W} \varrho_\mathrm{L} A v^2
$$
Air resistance is therefore not friction in the strict physical sense, as it depends on velocity. The quadratic dependence on $v$ explains both the increased fuel consumption at high driving speeds and the fact that skydivers eventually reach a terminal velocity.