Density & Pressure
📋 Module Overview & Key Units
General Learning Outcomes:
- 5.1 Use the following units: degree Celsius (°C), Kelvin (K), joule (J), kilogram (kg), kg/m³, metre (m), m², m³, m/s, m/s², newton (N), and pascal (Pa).
This module covers the fundamental concepts of density and pressure, including how pressure behaves in fluids. Correct units are crucial for all calculations.
⚖️ 1. Density
Learning Outcomes:
- 5.3 Know and use the relationship between density, mass, and volume.
- 5.4 Practical: Investigate density using direct measurements of mass and volume.
Density measures how compact a substance is — how much mass is packed into a given volume. A dense material (like iron) has a lot of mass in a small space, while a less dense material (like wood) is lighter for the same size.
ρ = m ÷ V
Density (kg/m³) = Mass (kg) ÷ Volume (m³)
Density = 400 g ÷ 50 cm³ = 8 g/cm³ = 8 000 kg/m³
📊 Common Material Densities
| Material | Density (kg/m³) | Float or Sink? |
|---|---|---|
| Cork | 240 | 🟢 Floats |
| Wood (oak) | 600 | 🟢 Floats |
| Ice | 917 | 🟢 Floats |
| Water | 1 000 | — |
| Aluminium | 2 700 | 🔴 Sinks |
| Iron | 7 874 | 🔴 Sinks |
| Copper | 8 960 | 🔴 Sinks |
| Gold | 19 320 | 🔴 Sinks |
🌊 Simulation: Float or Sink?
Click a material to drop it into the water tank and see whether it floats or sinks. Objects with density < 1 000 kg/m³ float; objects with density ≥ 1 000 kg/m³ sink.
🧮 Density Calculator
🔬 Practical: Measuring Density
- Electronic balance
- Regular-shaped objects (blocks, cylinders)
- Irregular-shaped objects (stones, metal samples)
- Ruler or vernier calipers
- Eureka can / displacement vessel
- Measuring cylinder, water, string
📐 Method A — Regular-Shaped Objects
🌊 Method B — Irregular-Shaped Objects (Displacement)
ρ = 50 g ÷ 20 cm³ = 2.5 g/cm³ = 2 500 kg/m³
💪 2. Pressure
Learning Outcomes:
- 5.5 Know and use the relationship between pressure, force, and area: P = F ÷ A.
Pressure is the force applied perpendicular to a surface, divided by the area over which it acts. The same force spread over a smaller area creates much higher pressure.
P = F ÷ A
Pressure (Pa) = Force (N) ÷ Area (m²)
P = 500 ÷ 0.25 = 2 000 Pa (2 kPa)
🔧 Simulation: Pressure vs Area
The same force is applied to both surfaces. Drag the sliders to see how changing force or area affects pressure. The colour intensity shows pressure level.
🧮 Pressure Calculator
🌊 3. Pressure in Fluids
Learning Outcomes:
- 5.6 Understand how pressure at a point in a fluid at rest acts equally in all directions.
- 5.7 Know and use the relationship: p = h × ρ × g.
In fluids (liquids and gases), pressure at any given point acts equally in all directions. This is called Pascal's Principle. Pressure also increases with depth because of the weight of fluid above.
At any point in a fluid at rest, pressure pushes equally in every direction. This is why submarines must be uniformly strong on all sides, and why water leaks from a hole in any direction.
p = h × ρ × g
Pressure difference (Pa) = height (m) × density (kg/m³) × gravitational field strength (m/s²)
p = 5 × 1 000 × 9.8 = 49 000 Pa = 49 kPa
🤿 Simulation: Depth Pressure Explorer
Drag the depth slider to move the submarine deeper. Watch how pressure increases with depth. Try different fluids!
⚙️ Simulation: Hydraulic Press
A hydraulic press uses Pascal's Principle: pressure applied to one piston is transmitted equally to the other. A larger output piston creates a larger force — a mechanical advantage!
🧮 Fluid Pressure Calculator
- Submarine hulls must withstand pressure from all sides equally.
- Water leaks through pipe holes in every direction.
- Deep-sea creatures evolved to withstand enormous equal pressure.
- Hydraulic brakes in cars transmit force through fluid pressure.