Mass and Density Explained Simply
Mass is how much "material" is packed into an object (measured in kg). Density describes how tightly that material is packed together - that is, mass per volume.
đź’¬ Mia asks Grandpa Theo
A block of wood and a block of iron can be the same size - so why is one so much heavier?
Because iron packs far more mass into the same space! Iron has a density of about 7.9 g/cm³, while wood is usually only 0.5-0.8 g/cm³ - roughly ten times less.
Is that why wood floats and iron doesn't?
Exactly! Water is 1.0 g/cm³. Anything less dense than water floats on top (like wood), anything denser sinks (like iron) - no matter how big or heavy the piece is overall.
A stone has a mass of 300 g and a volume of 120 cm³. What is its density?
ρ = 300 / 120 = 2.5 g/cm³ - heavier than water, so it would sink.
Large steel ships still float because you don't just count the metal - you count the whole ship with its hollow, air-filled interior. The average density of the entire ship ends up below that of water. The same idea, seen from the other side, is behind water displacement.
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