⇒ The density of a material is a measure of the mass per unit volume.

⇒ It is given the symbol, p

⇒ Density is a useful quantity because it allows us to compare different materials

⇒ The large stainless steel globe is more massive than the small stainless steel ball bearing. The globe floats; the ball bearing does not. Why?

⇒ Aerogels have the lowest density of any solid. The following table compares the density of aerogel with other common materials

⇒ Aerogel has been used in aerospace industries as a very low-density insulation material

• Aerogel is a good insulator because it traps air and prevents it from moving (if air can circulate it will transfer heat through convection currents)
• NASA used aerogel on the Mars landers Spirit and Opportunity to provide thermal insulation of key components

⇒ The NASA Startdust mission used aerogel to capture particles from the tail of comet Wild 2

• The very low density of the aerogel ensured that the comet dust particles were slowed down without being damaged
• The insulating properties of aerogel also meant that the dust particles were protected from the heat generated during re-entry of the sample collector into the Earth's atmosphere

⇒ Also see our notes on:

• Introduction
• Hooke's Law
• Stress and the Young Modulus