Sink or float: buoyancy

What the experiment shows

Water pushes a body up with a force equal to the weight of the displaced water. If the body’s average density is lower than water’s it floats, if higher it sinks.

How to run it

1. Press “Run” and watch the bodies fall into the water.
2. Notice how deep each body floats: the lighter, the higher.
3. In the editor change the brick’s mass (to 15 kg) — does it float?

Questions to discuss

• Why does a steel ship not sink although steel is heavier than water?
• How does a submarine dive and surface?

The science

The buoyant force is F = ρ·g·V, where V is the submerged volume. A body floats when F balances its weight m·g, i.e. the submerged fraction is ρbody/ρwater. A ship is light on average: it is full of air.

Try it at home

Fill a basin with water and drop a cork, a coin, an apple and a clay ball. Then shape the clay into a boat — it floats!

What to change in the editor

• Change the fluid density: 1.2 is salt water.