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)

This section focuses on understanding the force of drag, which opposes the motion of an object through a fluid – whether that fluid is a liquid or a gas. Drag is a type of friction, but specifically refers to the friction experienced by an object moving through a fluid.

Understanding Drag

Drag is a force that acts in the opposite direction to the motion of an object through a fluid. It's caused by the fluid having to be pushed out of the way as the object moves. The magnitude of the drag force depends on several factors, including the speed of the object, the shape of the object, and the density of the fluid.

Factors Affecting Drag

The magnitude of the drag force is affected by the following:

• Speed: Drag force generally increases with speed. The faster the object moves, the greater the drag.
• Shape: A streamlined shape experiences less drag than a shape with a large surface area perpendicular to the direction of motion.
• Size: Larger objects generally experience more drag.
• Fluid Density: Drag force increases with the density of the fluid. Air is less dense than water, so an object will experience less drag in air than in water (assuming the same shape and speed).

Drag Equation

The magnitude of the drag force can be approximated using the following equation:

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

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 (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 (perpendicular to the direction of motion, in m²)

Examples of Drag

We experience drag in many everyday situations:

• Air Resistance: When a skydiver jumps, the air resistance (drag) slows them down. Streamlining their body reduces the amount of drag.
• Car Speed: The shape of a car is designed to reduce air resistance, allowing it to travel faster with less fuel consumption.
• Cycling: Wearing aerodynamic clothing and a helmet can reduce the drag experienced by a cyclist.
• Swimming: Swimmers adopt streamlined positions to minimize drag and improve their speed.

Suggested Diagram:

Factor	Effect on Drag Force
Speed	Increases (proportional to the square of the speed)
Shape	Decreases (more streamlined shapes have lower drag)
Size	Increases
Fluid Density	Increases