Skip to main content
Physics LibreTexts

9.4: Some Functions of the Masses

In section 9.5 I am going to consider the motion of two masses, \(M\) and \(m\) around their mutual centre of mass under the influence of their gravitational attraction. I shall probably want to make use of several functions of the masses, which I shall define here, as follows:

Total mass of the system: 

\[\textbf{M} = M + m. \label{9.4.1} \tag{9.4.1}\]

"Reduced mass" \[\text{m} = \frac{Mm}{M + m} . \label{9.4.2} \tag{9.4.2}\]

"Mass function":\[\mathfrak{M} = \frac{M^3}{(M+m)^2} . \label{9.4.3} \tag{9.4.3}\]

No particular name: \[m_+ = m \left( 1 + \frac{m}{M} \right) . \label{9.4.4} \tag{9.4.4}\]

Mass ratio: \[q = m/M . \label{9.4.5} \tag{9.4.5}\]

Mass fraction: \[ \mu = m/(M+m) . \label{9.4.6} \tag{9.4.6}\]

The first four are of dimension M; the last two are dimensionless. When \(m << M\),  \(\text{m} → m\), \(\mathfrak{M} → M\) and \(m_+ → m\).