mechanical · 28 minutes · Official OETT Aligned
Density & Floating
Why some objects float and others sink
What to know
- density: mass packed into a given space
- volume: space an object takes up
- buoyancy: upward push from a liquid
- water pushed aside
A heavy object can float, and a small light object can sink.
The core rule
Compare average density, not weight alone. An object floats when the liquid can push up enough to balance its weight.
Density means mass divided by volume. Volume is the space an object takes up. Liquid pushes upward on an object; this is buoyancy. An object pushes liquid out of the space it occupies. A hollow boat spreads its mass over a large volume, so its average density can be lower than water even if its hull is steel.
Worked examples
Example 1: easy
Equal-volume blocks: the lighter block has lower density.
- Both blocks take up the same amount of space.
- The lighter block has less mass in that space, so its density is lower.
Density = mass ÷ volume. With equal volume, less mass means lower density.
Example 2: medium
A hollow steel hull can float though solid steel sinks.
- A solid steel piece is denser than water and sinks.
- A hollow hull includes a large space inside, lowering its average density enough to float.
Compare the average density of the whole object, including its hollow space, with water.
Example 3: hard
Adding sealed space without much extra weight can help an object float.
- Add sealed space without adding much weight.
- The object can now push aside more water, allowing a larger upward push.
More sealed volume lowers average density and can help the same weight float.
Common mistakes
Every heavy object sinks and every light object floats.
Floating depends on average density and how much liquid is pushed aside, not just total weight.
Put it into practice
Use guided explanations, check your answers, and try a new variation.