Packed Tight or Loose?
Density — why iron sinks and wood floats
What this lesson covers
Why it matters
A tiny iron nail sinks instantly. A whole wooden log floats. It's not about big or small, heavy or light — it's about how TIGHTLY the stuff inside is packed.
Predict first
Wood floats on water because wood is…
Float or sink is a density contest: less dense than water floats, denser sinks. A giant log floats; a tiny nail sinks.
- less dense than water — correct
- lighter than every iron object
- dry
What you do
Load the floating barge crate by crate — feel how adding mass (without adding size) pushes its density toward sinking.
Check yourself
Density = …
How much stuff per space: ρ = m/V. Water's is 1 g/cm³ — the referee.
Oil floats on water because oil is…
≈0.8 g/cm³ vs water's 1 — the lighter liquid takes the top floor.
An iron SHIP floats because…
Iron + lots of air inside a huge hull = average density below 1 — the nail never had that trick.
- mass ÷ volume — correct
- mass × volume
- weight ÷ size of surface
- less dense than water — correct
- slippery
- warmer
- its hollow shape makes its average density less than water's — correct
- ships are painted
- the sea is extra strong