Drag and friction
Forces in Action - OCR A-Level Physics
Key Definition
Friction
A resistive contact force exerted by objects or materials sliding past one another (kinetic friction) or trying to slide past one another (static friction). It always acts opposite to the direction of motion or attempted motion.
A resistive contact force exerted by objects or materials sliding past one another (kinetic friction) or trying to slide past one another (static friction). It always acts opposite to the direction of motion or attempted motion.
Drag
Drag
The friction force exerted on an object moving through a fluid (liquid or gas). When the fluid is air, drag is also called air resistance. Drag always opposes relative motion through the fluid.
The friction force exerted on an object moving through a fluid (liquid or gas). When the fluid is air, drag is also called air resistance. Drag always opposes relative motion through the fluid.
$$F_{\text{drag}} \propto v^{n} \quad (n \approx 1 \text{ at low speed},\ n \approx 2 \text{ at high speed})$$
- Drag is larger when the surface area facing the flow is larger; that is why a parachute slows a skydiver so effectively.
- Drag is larger at higher relative speed. A car at $30 \text{ m s}^{-1}$ experiences roughly four times the drag of one at $15 \text{ m s}^{-1}$ if drag is proportional to $v^{2}$.
- Drag is larger in a more viscous fluid (e.g. oil $>$ water $>$ air).
- Drag is smaller for a more aerodynamic (streamlined) shape, because the airflow stays attached and there is less turbulent wake.
- Static friction can take any value up to a maximum (the "limiting" static friction). Once sliding begins, kinetic friction is usually slightly less than the limiting static value.
Diagram pending
Sphere falling through a viscous fluid. Three arrows on the sphere: weight $W$ down, drag $F_{\text{drag}}$ up (in direction opposite to motion), upthrust $U$ up. Labelled streamlines around the sphere.
Will be replaced with a GeoGebra SVG in stream 2.
Common Mistake
MEDIUM
Students often: assume drag stays constant as an object speeds up.
Instead: drag grows with speed. That is the mechanism behind terminal velocity: drag rises until it matches the weight, so the resultant force falls to zero.
Instead: drag grows with speed. That is the mechanism behind terminal velocity: drag rises until it matches the weight, so the resultant force falls to zero.
Examiner Tips and Tricks
- When asked to LIST factors affecting drag, give all four: surface area, speed, fluid viscosity, shape (streamlining).
- State that drag opposes the direction of motion through the fluid. Markers reject "drag opposes gravity" because that is only true for a falling object.