# Help Zone

### Student Question

Hello! Must I measure a solid with a ruler to determine its density ? I no longer remember :(

Science ## Explanations (1)

• Explanation from Alloprof  Explanation from Alloprof

This Explanation was submitted by a member of the Alloprof team.

Hello Fast Fox!

One can measure the dimensions of a solid whose shape is the same as that for which a volume formula is known, such as a sphere, a cylinder or a rectangular prism. These dimensions can then be used to calculate the volume with the appropriate volume formula.

However, very few solids have a sufficiently regular shape for this method to be accurate. Their volume must thus be measured indirectly.

A graduated cylinder with as fine graduations as possible has to be used in order to do this. It must be wide enough to contain the solid being studied. One must then add enough water in the cylinder to cover the solid (without the solid being in the cylinder yet) and take note of the volume of water present. Finally, the solid has to be placed in the graduated cylinder, causing the water in the cylinder to rise. The final volume (which must be taken note of, of course) measures the combined volume of the initial water and the solid.

To know the solid’s volume, the volume of water must be subtracted from the combined volume of water and solid, as in the formula below:

$$V_{solid} = V_{water + solid} − V_{water}$$

As density is a ratio of mass contained for a given volume, the solid must also be weighed. A beam balance or electronic balance can thus be used.