Letter: Demonstrating the density illusion of different weight (2)

From Kevin Privett,Llandough, Vale of Glamorgan, UK

Yes, density does affect our perception of weight. When I worked in concrete technology, I was told about a laboratory open day for which staff made small concrete cube specimens using different mixes.

At one extreme, they replaced the coarse gravel aggregate with lead shot and the fine sand with crushed barium sulphate; similar mixes are used in concrete for radiation shielding. At the other extreme, the aggregate was polystyrene beads and the fine aggregate hollow glass spheres from power station ash. These cubes floated on water.

All looked like regular concrete, as did cubes of intermediate density. Visitors who picked a high-density cube initially couldn’t move it, because it was unexpectedly heavy. Those choosing a low-density cube felt their arms shoot upwards because it was unexpectedly light.