IGCSE Physics 0625

1.5.1 Effects of Forces

Objective: Know that friction (drag) acts on an object moving through a gas (e.g. air resistance)

When an object moves through a gas, such as air, it experiences a force opposing its motion. This force is called drag or air resistance. Drag is a type of friction, but it acts specifically on objects moving through fluids (liquids and gases).

What causes Drag?

Drag is caused by the object displacing the gas molecules. As the object moves, it has to push the gas molecules out of the way. This requires energy, and this energy transfer results in a force acting opposite to the direction of motion.

Factors Affecting Drag

The magnitude of the drag force depends on several factors:

• Speed: The faster the object moves, the greater the drag force. The drag force is generally proportional to the square of the speed.
• Shape: Streamlined shapes experience less drag than blunt shapes. A streamlined shape allows the gas to flow smoothly around the object.
• Size: Larger objects generally experience more drag.
• Density of the gas: Denser gases exert more drag than less dense gases. For example, an object moving through water will experience more drag than the same object moving through air.
• Surface area: A larger surface area exposed to the moving gas will result in greater drag.

Mathematical Representation of Drag

The drag force (F_d) can be approximated by the following equation:

$$F_d = \frac{1}{2} \times \rho \times v^2 \times C_d \times A$$

Where:

• $\rho$ is the density of the gas (kg/m³)
• $v$ is the speed of the object relative to the gas (m/s)
• $C_d$ is the drag coefficient (a dimensionless number that depends on the shape of the object)
• $A$ is the cross-sectional area of the object (m²) perpendicular to the direction of motion

Examples of Drag

• Air resistance on a falling object: A feather falls slowly because its large surface area creates a significant drag force. A stone falls quickly because its smaller surface area results in less drag.
• Wind resistance on a car: Aerodynamic car designs have a low drag coefficient, allowing them to travel at higher speeds with less energy consumption.
• Swimming: Swimmers try to streamline their bodies to reduce drag and improve their speed.

Factor	Effect on Drag
Speed	Increases significantly (proportional to the square of the speed)
Shape	Streamlined shapes reduce drag; blunt shapes increase drag
Size	Larger objects generally experience more drag
Gas Density	Denser gases increase drag
Surface Area	Larger surface area increases drag