IGCSE Physics - 1.5.1 Effects of Forces

Friction (Drag)

When an object moves through a liquid (like air or water), it experiences a force that opposes its motion. This force is called drag or fluid friction. Drag acts in the opposite direction to the object's velocity.

What causes drag?

Drag is caused by the interaction between the fluid molecules and the surface of the object. As the object moves, it pushes the fluid molecules out of the way. This creates areas of higher and lower pressure around the object. The pressure difference results in a net force opposing the motion – the drag force.

Factors affecting drag

The magnitude of the drag force depends on several factors:

• Speed of the object: The faster the object moves, the greater the drag force. Drag force is approximately proportional to the square of the velocity.
• Shape of the object: Streamlined shapes experience less drag than blunt shapes.
• Size of the object: Larger objects generally experience more drag.
• Viscosity of the fluid: More viscous fluids (like honey) create more drag than less viscous fluids (like water).
• Surface area of the object: A larger surface area exposed to the fluid will result in greater drag.

Mathematical Representation of Drag Force

The drag force can be approximated by the following equation:

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

Where:

• $F_d$ is the drag force (in Newtons, N)
• $\rho$ is the density of the fluid (in kg/m³)
• $v$ is the speed of the object relative to the fluid (in 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 (in m²) perpendicular to the direction of motion.

Examples of Drag

1. Air resistance on a falling object: A feather falls slowly due to its large surface area and relatively high drag coefficient. A streamlined object like a dart falls much faster.
2. Resistance to a boat moving through water: A streamlined hull reduces drag, allowing the boat to move more efficiently.
3. Friction experienced by a cyclist: A cyclist reduces drag by adopting a streamlined position.

Factor	Effect on Drag Force
Speed	Increases (approximately proportional to $v^2$)
Shape	Streamlined shape decreases drag, blunt shape increases drag
Size	Larger object increases drag
Viscosity	Higher viscosity increases drag
Surface Area	Larger surface area increases drag