A rigid bodyRigid body is a system of particles such that the distances between any pair are fixed. For instance, we could have four mass points \(m_a\) for \(a = 1,2,3,4\) arranged at the vertices of a regular tetrahedronTetrahedron and pairwise connected by rigid rods of negligible mass as shown in Fig. 8.1a. A continuous distribution of mass is also possible, as in a stone (see Fig. 8.1b). In this case, the discrete indexIndexdiscrete a is replaced with the continuous location ‘index’ \({\boldsymbol{r}}\) and the masses of individual particles are replaced by the mass \(dm({\boldsymbol{r}}) = \rho ({\boldsymbol{r}}) dV\) in any elemental volume dV around \({\boldsymbol{r}}\) where \(\rho ({\boldsymbol{r}})\) is the local mass densityMassdensityDensitymass. Many commonly encountered objects can be treated as rigid bodies. Examples include a point mass, a dumbbell, a plate, a cricket or golf ball, a boomerang, a top, a gyroscope, an asteroid and a spaceship. The concept is useful in quantum physics as well; e.g., the rotational energy levelsEnergyspectrumrotational (as opposed to the vibrational spectrumEnergyspectrumvibrational) of a moleculeMolecule can be approximately understood by treating it as a rigid body.