The amount of space that a substance takes up is called its volume. It is measured in cubic units. For example, a cube measuring one metre on each side has a volume of one cubic metre or 1m3. But a cubic metre of lead has a much greater mass than a cubic metre of wood. That is because the lead has a much higher density than the wood. Its particles are more tightly packed together. The density of an object is calculated by dividing its mass by its volume and is expressed as kilograms per cubic metre (kg/m3) or pounds per cubic foot (lb/ft3).
Density is a measure of how compact the mass in a substance or object is. The density of an object or substance can be calculated from this equation: density in kilograms per meter cubed is equal to mass in kilograms, divided by volume in meters cubed. Or in other words, density is mass spread out over a volume. Or in other, other words, it’s the number of kilograms that 1 meter cubed of the substance weights. If each meter cubed weighs more, the substance is denser.
As we’ll discuss in other lessons, density is super important because it relates to whether things rise or sink. Less dense materials tend to rise above more dense materials, particularly in the case of liquids and gases. So understanding density has major implications for the motions of materials and gases in the atmosphere and objects floating (or sinking) in water. Density is the reason some objects sink and other objects float. And it’s the reason that some clouds are high in the sky, while others are low down.
Density means that if you take two cubes of the same size made out of different materials and weigh them, they usually won’t weigh the same. It also means that a huge cube of Styrofoam can weigh the same as a tiny cube of lead.
Examples of dense materials include iron, lead, or platinum. Many kinds of metal and rock are highly dense. Dense materials are more likely to ‘feel’ heavy or hard. Although a sparse material (sparse is the opposite of dense) can feel heavy if it’s really big. Examples of sparse materials would be Styrofoam, glass, soft woods like bamboo, or light metals like aluminum.
In general, gases are less dense than liquids and liquids are less dense than solids. This is because solids have densely-packed particles, whereas liquids are materials where particles can slide around one another, and gases have particles free to move all over the place.
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