Knowledge Organiser: Density
Key Terms
- Density (ρ): mass per unit volume
- Displacement: volume of fluid pushed aside by an object
- Upthrust: upward force on a submerged object
- Regular solid: shape with calculable volume (cube, cylinder)
- Irregular solid: shape needing displacement method
Key Facts
- Density is a property of the material, not the object size
- Water density = 1000 kg/m³ = 1 g/cm³ (benchmark)
- Objects float if density < fluid density
- Objects sink if density > fluid density
- Gases are ~1000× less dense than solids/liquids
Key Equations
- ρ = m / V (density = mass ÷ volume)
- Units: kg/m³ or g/cm³
- Conversion: 1 g/cm³ = 1000 kg/m³
- Displacement volume: V = V₂ − V₁
- Higher: P = ρgh (fluid pressure)
Exam Tips
- Always show units in density calculations
- For displacement: volume = final − initial reading
- Air bubbles on object = source of error in practical
- Cutting a material in half does NOT change its density
Common Mistakes
- Confusing mass and density: A larger object is not necessarily denser — density depends on mass per unit volume, not total mass
- Wrong units in calculations: If mass is in g and volume in cm³, density is in g/cm³ — convert to kg and m³ if SI units are required
- Not subtracting initial volume in displacement: When using a measuring cylinder, volume of object = final reading − initial reading — do not use the final reading alone
- Saying density changes when you cut a material: Density is an intrinsic property — cutting a block in half halves both mass and volume, so density stays the same
- Air bubbles in the displacement practical: Air bubbles on the object's surface add extra apparent volume — this is a source of error that makes density appear too low
Memory Aid
- Cover ρ: density = m ÷ V
- Cover m: mass = ρ × V
- Cover V: volume = m ÷ ρ