Float or sink?
Show on a big screenA pebble sinks in a pond. A dry leaf floats. A steel ship floats too. What decides?
- A tank of water. Water has a density of 1 g/cm³.
- A block of density 0.6 floats, with part above the water.
- A block of density 1 hangs in the middle.
- A block of density 2.7 sinks to the floor.
- Less than 1 floats. More than 1 sinks.
The rule
Density decides. Compare the object's density with the liquid's. The liquid matters too. Very salty water is denser than tap water. Denser means more mass in each cm³. Dead Sea water has a density of about 1.24 g/cm³.
Rule: An object floats if its density is less than the liquid's. It sinks if its density is more.
Same density: the object stays where you leave it. A hollow object has empty space inside. Use its whole mass and whole volume.
| Material | Density in g/cm³ | In water |
|---|---|---|
| Light wood (made-up) | 0.6 | floats |
| Cooking oil | 0.9 | floats |
| Water | 1 | hangs |
| Aluminium | 2.7 | sinks |
| Iron | 7.9 | sinks |
Watch out!
Steel is denser than water, yet a steel ship floats. The ship holds air inside. So the whole ship has a density below water's.
Divide first. Then compare with the liquid.
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Try the lab to finish this lesson.