Cambridge IGCSE Physics 0625 – Effects of Forces: Friction (Drag) in Liquids
1.5.1 Effects of Forces – Friction (Drag) in Liquids
Learning Objective
Know that friction (drag) acts on an object moving through a liquid and be able to describe the factors that influence its magnitude.
Key Definitions
Friction: A force that opposes the relative motion of two surfaces in contact.
Drag: The form of friction experienced by an object moving through a fluid (liquid or gas). In this context we focus on liquids.
Viscosity: A measure of a fluid’s internal resistance to flow; higher viscosity means greater drag.
Why Drag Occurs
When an object moves through a liquid, its surface pushes against the liquid molecules. The liquid molecules, in turn, exert an opposite force on the object. This opposite force is the drag force, which always acts in the direction opposite to the motion.
Factors Affecting Drag
The magnitude of the drag force depends on several variables. The relationship can be expressed by the empirical equation:
\$Fd = \frac{1}{2} Cd \,\rho \,A \,v^{2}\$
where:
\$F_d\$ = drag force (N)
\$C_d\$ = drag coefficient (dimensionless, depends on shape)
\$\rho\$ = density of the liquid (kg m⁻³)
\$A\$ = cross‑sectional area of the object perpendicular to the flow (m²)
\$v\$ = speed of the object relative to the liquid (m s⁻¹)
Factor
Effect on Drag
Speed of the object (\$v\$)
Drag increases with the square of speed; doubling speed quadruples drag.
Cross‑sectional area (\$A\$)
Larger area presents more liquid to push against, increasing drag proportionally.
Liquid density (\$\rho\$)
Denser liquids (e.g., water vs. oil) produce greater drag.
Shape (drag coefficient \$C_d\$)
Streamlined shapes have low \$Cd\$, reducing drag; blunt shapes have high \$Cd\$.
Viscosity of the liquid
Higher viscosity (thicker liquids) increases resistance, especially at low speeds.
Common Everyday Examples
A swimmer feels resistance as they move through water; the faster they swim, the greater the drag.
A falling raindrop reaches a constant speed (terminal velocity) when the drag force equals its weight.
When a boat moves, its hull shape is designed to minimise drag, allowing higher speeds with less engine power.
A skydiver in a spread‑eagle position experiences more drag than when they tuck into a streamlined position.
Misconceptions to Address
“Drag only occurs in air.” – Drag is present in any fluid, including water, oil, and even honey.
“Faster motion always means more friction.” – In liquids, the increase is quadratic with speed, not linear.
“All objects of the same size experience the same drag.” – Shape and surface texture (affecting \$C_d\$) are crucial.
Practical Investigation Idea
Investigate how the shape of an object influences drag in water.
Prepare three objects of equal mass and volume but different shapes (e.g., a sphere, a flat plate, and a streamlined body).
Attach each to a spring balance and pull them through a water tank at a constant speed using a pulley system.
Record the force reading for each shape; the higher the reading, the greater the drag.
Discuss how the results relate to the drag coefficient \$C_d\$.
Summary
Drag is a type of friction that opposes motion through a liquid.
Its magnitude depends on speed, area, liquid density, shape, and viscosity.
The drag force follows the equation \$Fd = \frac{1}{2} Cd \rho A v^{2}\$, showing a quadratic relationship with speed.
Understanding drag helps explain everyday phenomena and is essential for designing efficient vehicles and equipment.
Suggested diagram: An object moving through water with arrows indicating the direction of drag opposite to motion, and labels for speed \$v\$, cross‑sectional area \$A\$, and drag force \$F_d\$.