Cambridge IGCSE Physics 0625 – 1.5.1 Effects of Forces
1.5.1 Effects of Forces
Objective
Know that friction (drag) acts on an object moving through a gas (e.g. air resistance) and understand the factors that influence its magnitude.
Key Concepts
Drag (air resistance) is a type of friction that occurs when an object moves through a fluid such as air.
It always acts opposite to the direction of motion, reducing the object's speed unless an external force compensates for it.
Drag is not constant; it depends on several variables described below.
Factors Affecting Drag
Factor
How it influences drag
Typical IGCSE example
Speed of the object (\$v\$)
Drag increases rapidly with speed; at high speeds it is proportional to \$v^{2}\$.
Skydiver falling faster experiences greater air resistance.
Cross‑sectional area (\$A\$)
Larger area presents more surface for the fluid to act on, increasing drag.
A flat sheet of paper falls slower than a rolled piece of paper of the same mass.
Shape (drag coefficient \$C_{d}\$)
Streamlined shapes have lower \$C{d}\$, reducing drag; blunt shapes have higher \$C{d}\$.
Airplane wings are shaped to minimise drag.
Density of the fluid (\$\rho\$)
Denser fluids exert more drag for the same speed and area.
Moving through water (high \$\rho\$) feels much more resistance than moving through air.
Viscosity of the fluid
Higher viscosity increases the frictional component of drag, especially at low speeds.
Oil (high viscosity) slows a moving object more than air.
Mathematical Description of Drag
Two common approximations are used in the IGCSE syllabus:
Linear (low‑speed) approximation – when speeds are small, drag is roughly proportional to speed:
\$F_{d}=k\,v\$
where \$k\$ is a constant that depends on the object's shape, area and the fluid’s properties.
Quadratic (high‑speed) approximation – for faster motion, drag varies with the square of the speed:
\$F{d}= \frac{1}{2}\,C{d}\,\rho\,A\,v^{2}\$
\$C_{d}\$ – drag coefficient (dimensionless, depends on shape)
\$\rho\$ – density of the fluid (kg m⁻³)
\$A\$ – cross‑sectional area perpendicular to the motion (m²)
\$v\$ – speed of the object relative to the fluid (m s⁻¹)
Comparing Drag with Surface Friction
Both are resistive forces, but they differ in origin and behaviour:
Surface friction occurs between two solid surfaces in contact. Its magnitude is largely independent of speed (kinetic friction) and depends on the nature of the surfaces and the normal force.
Drag acts between a solid object and a fluid (gas or liquid). Its magnitude depends strongly on speed, area, shape and fluid properties.
Practical Implications for IGCSE Experiments
When measuring the acceleration of a falling object, air resistance reduces the observed acceleration compared with \$g\$.
In a “paper parachute” experiment, increasing the parachute’s area or using a more porous material increases drag and slows the descent.
Ballistic trajectories are altered by drag; the range is shorter than the ideal vacuum prediction.
Suggested diagram: A falling object with arrows indicating the weight \$mg\$ downward and the drag force \$F_{d}\$ upward, showing dependence on speed.
Summary Checklist
Drag always opposes motion through a fluid.
At low speeds, drag ≈ \$k v\$; at higher speeds, drag ≈ \$\tfrac12 C_{d}\rho A v^{2}\$.