Drag is a special kind of friction that acts on an object when it moves through a gas (like air). Unlike the friction you feel on a floor, drag depends on the shape of the object, how fast it moves, and the density of the gas. Think of it as the invisible hand that tries to slow you down when you fly, run, or drive.
- It limits the speed of a skydiver before the parachute opens.
- It determines how much fuel a car needs to keep moving.
- It explains why a bicycle rider feels a breeze when they pedal faster.
The magnitude of the drag force is given by:
\$Fd = \tfrac{1}{2} Cd \, \rho \, A \, v^2\$
| Shape | Typical \$C_d\$ |
|---|---|
| Sphere (smooth) | 0.47 |
| Cube (sharp edges) | 1.05 |
| Flat plate (edge facing flow) | 1.28 |
| Streamlined body (e.g., car) | 0.30–0.35 |
- Drag is a form of friction that acts opposite to the direction of motion.
- It depends on the square of the speed (\$v^2\$), so doubling the speed quadruples the drag force.
- Reducing the drag coefficient or the reference area can significantly lower the drag force, which is why aerodynamic design matters in sports, transport, and engineering.