Drag force is a resistive force experienced by an object moving through a fluid (such as air or water). It acts opposite to the direction of the object's motion and depends on factors like velocity, fluid density, and cross-sectional area.
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Drag force increases with the square of the object's velocity: $F_d = \frac{1}{2} C_d \rho A v^2$ where $C_d$ is the drag coefficient, $\rho$ is the fluid density, $A$ is the cross-sectional area, and $v$ is velocity.
The drag coefficient ($C_d$) is a dimensionless number that depends on the shape and surface roughness of the object.
For low velocities in viscous fluids, drag force can be approximated using Stokes' law: $F_d = 6 \pi \eta r v$, where $\eta$ is the dynamic viscosity, $r$ is the radius of a spherical object, and $v$ is velocity.
Terminal velocity occurs when drag force equals gravitational force, resulting in zero net acceleration.
Drag forces are crucial in designing vehicles and aircraft to minimize fuel consumption and optimize performance.