Dense Sense

Why it matters

A kilogram of gold fits in a matchbox; a kilogram of feathers fills a pillowcase. Density is the crowd-density of matter — and it's a fingerprint no metal can fake.

The idea in plain words

Matter per space. Tap the terms.

ρ = m ÷ V · R.D. = ρ ÷ ρ(water)

R.D. of a liquid is 0.8. Its density in kg/m³?

800 kg/m³ — lighter than water (it's oil, and yes it floats)

• g/cm³ or kg/m³: ρ — density — mass packed per unit volume; water: 1 g/cm³ = 1000 kg/m³
• dimensionless: R.D. — relative density — a pure ratio against water, NO units; R.D. 7.9 means 7.9× water
• next lesson: float rule preview — R.D. < 1 floats in water, R.D. > 1 sinks — density decides destiny
• ρ = R.D. × 1000

Predict first

A block has mass 79 g and volume 10 cm³. Its density says it is…

ρ = m/V = 79/10 = 7.9 g/cm³ — the textbook density of iron. Measure mass, measure displacement, and the table names your metal.

• 7.9 g/cm³ — iron — correct
• 790 g/cm³ — super-dense
• 0.13 g/cm³ — floats

What you do

Weigh, displace, divide, identify — two mystery blocks.

Check yourself

Relative density has…

Units cancel in the ratio — that's why R.D. is the same number in every unit system.

Ice (ρ = 0.92 g/cm³) floats on water with roughly…

The submerged fraction equals the density ratio — icebergs hide nine-tenths of themselves.

Which measurement pair identifies an irregular stone's density?

No formula fits a lump — Archimedes' overflow does. Eureka, again.

• no unit — it's a ratio of two densities — correct
• the unit g/cm³
• the unit newton
• 92% submerged, 8% above — correct
• 8% submerged
• half and half
• mass on a balance + volume by water displacement — correct
• length × breadth × height
• weight in air alone