Physics A Level
Chapter 7: Matter and materials 7.1 Density
Physics A Level
Chapter 7: Matter and materials 7.1 Density
SPRINGY STUFF
In everyday life, we make great use of elastic materials. The term ‘elastic’ means springy; that is, the material deforms when a force is applied and returns to its original shape when the force is removed.
Rubber is an elastic material. This is obviously important for a bungee jumper (Figure 7.1). The bungee rope must have the correct degree of elasticity. The jumper must be brought gently to a halt. What happens if the rope is too stiff or too springy? Discuss these problems with others – particularly if you have had experience of a bungee jump.
In this chapter, we will look at how forces can change the shape of an object. Before that, we will look at two important quantities, density and pressure.
Density is a property of matter. It tells us about how concentrated the matter is in a particular material.
Density is a constant for a given material under specific conditions.
Density is defined as the mass per unit volume of a substance:
$\begin{array}{l}
density = \frac{{mass}}{{volume}}\\
\rho = \frac{m}{V}
\end{array}$
The symbol used here for density, $\rho $, is the Greek letter rho.
The standard unit for density in the SI system is $kg\,{m^{ - 3}}$, but you may also find values quoted in $g\,c{m^{ - 3}}$. It is useful to remember that these units are related by:
$1000\,kg\,{m^{ - 3}} = 1\,g\,c{m^{ - 3}}$
and that the density of water is approximately $1000\,kg\,{m^{ - 3}}$.
Questions
1) A cube of copper has a mass of $240 g$. Each side of the cube is $3.0 cm$ long. Calculate the density of copper in $g\,c{m^{ - 3}}$ and in $kg\,{m^{ - 3}}$.
2) The density of steel is $7850\,kg\,{m^{ - 3}}$. Calculate the mass of a steel sphere of radius $0.15 m$. (First, calculate the volume of the sphere using the formula $V = \frac{4}{3}\pi {r^3}$ and then use the density equation.)