Drag force

Definition

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.

5 Must Know Facts For Your Next Test

1. 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.
2. The drag coefficient ($C_d$) is a dimensionless number that depends on the shape and surface roughness of the object.
3. 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.
4. Terminal velocity occurs when drag force equals gravitational force, resulting in zero net acceleration.
5. Drag forces are crucial in designing vehicles and aircraft to minimize fuel consumption and optimize performance.

Review Questions

• How does drag force depend on an object's velocity?
• What factors affect the drag coefficient ($C_d$)?
• Explain how terminal velocity is achieved for a falling object.

Related terms

Friction: A resistive force that opposes relative motion between two surfaces in contact. It acts parallel to the surfaces.

Terminal Velocity: The constant speed that a freely falling object eventually reaches when the resistance from the medium prevents further acceleration.

Newton's Third Law: $\text{For every action, there is an equal and opposite reaction.}$ This principle explains interactions between objects and forces.