Density and buoyancy game: predict, drop it and watch it fall
A physics game for really understanding floating, not memorising it. A real object (cork, ice, an apple, an egg, a marble, a coin, a piece of gold) is held above a glass of liquid, which may be water, seawater, olive oil, alcohol, glycerine, honey, Dead Sea water or mercury. You predict what will happen when it is dropped. When you answer, the object falls and stays exactly where physics says: floating with just the right part under the surface, or on the bottom of the glass. On easy you see each object’s density; on medium only its mass and volume, so you work the density out; on hard you are asked how much stays under the surface.
Physics · Primary, Secondary
Most children believe heavy things sink and light things float. A huge log floats and a tiny marble sinks: what counts is density, the mass packed into each cubic centimetre. The game tests it again and again with objects of every size until the idea replaces the intuition.
An egg sinks in tap water and floats in the Dead Sea; ice floats in water and sinks in alcohol; even lead floats on mercury. Changing the liquid forces you to compare two densities instead of sorting objects into “floaters” and “sinkers”, which is the underlying mistake.
On hard, the submerged part is the object’s density divided by the liquid’s: a 0.6 g/cm³ block in water floats with 60% under the surface. The numbers are chosen to come out exact, and the drawing shows them: the part tinted by the liquid is exactly that fraction.
"A 300 g, 500 cm³ block appears above a glass of water. Its density is 300 ÷ 500 = 0.6 g/cm³, less than water (1 g/cm³): it floats. How much is under the surface? 0.6 ÷ 1 = 60%. Many answer 40%, which is the part sticking out: the game includes that option on purpose. When dropped, the block falls and settles with three fifths under the water."
Prefer to learn without screens? Try this analogue version:
Fill a glass with tap water and gently lower a raw egg into it: it goes to the bottom.
Take it out and dissolve salt in the water, spoonful by spoonful, stirring well.
Put the egg back after each spoonful and note how many it takes for it to float.
Explain why: the salt raises the density of the water until it is higher than the egg’s.
If you like Float or Sink?, these activities work similar skills:
Covers pressure and area, hydrostatic pressure, Pascal’s and Archimedes’ principles and atmospheric pressure, with an explanation after each answer.
In a tall glass, slowly pour honey, coloured water and oil: they form layers. Then drop in small objects and see which layer each one stops at.
A ball of modelling clay sinks; the same clay shaped into a boat floats. Same mass, more volume: the average density drops.
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