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My kids enjoy playing with their LEGOs in the pool, and we cannot always predict which ones will float and which one will sink.

It appears to me that most pieces do float in the pool, but some, for instance a motorbike, doesn't.

How can I distinguish, if at all possible, which ones will float, before actually putting them in the pool?

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Most Lego pieces sink. Some of them will float briefly if they have air trapped inside. Almost any construction of Lego elements will sink as there are gaps between pieces.

There are some boat pieces that are designed to float, such as the Police Patrol Boat set from 2016. A Brickset list shows some other boats that float.

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Some fairly straightforward physics is your friend here:

To determine if something will sink or float in a given medium, you need to know its density. If the density of the object is less than the density of the medium it will float.

Standard pool water would have a density around 1g/cm3, salt water is higher (depends on the amount of salt in the water).

To calculate the density of the LEGO elements, you need to weigh it, and divide that by its volume.

The slight complication is as Mr Shiny and New states: Bricks can trap some air inside them which reduce their density while the air remains trapped in the brick. Items like the motorbike are almost solid ABS, and so are much more dense than a minifig with a hollow body, arms and legs.

  • This method quickly becomes complicated if one wishes to include the weight and volume of the air "inside" a boat piece, which clearly does float, but ignoring this amount from the calculation would probably indicate otherwise. – zovits Apr 17 at 13:19
  • Indeed - that's where displacement comes in (and how steel ships float). But it's the best way to tell whether an object will float or sink before you drop it into a pool. Even a boat element will sink if you flood it with water. – Zhaph - Ben Duguid Apr 17 at 13:39

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