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A dynamical system consists of three particles \(m_{1}, m_{2}, m_{3}\) in motion in space. The distance between \(m_{1}\) and \(m_{2}\), \(m_{2}\) and \(m_{3}\), \(m_{1}\) and \(m_{3}\) remain constant throughout the motion. The motion will be governed by \(n\) Euler Lagrange equations. Then :

  • A.\(n = 7\)
  • B.\(n = 6\)
  • C.\(n = 9\)
  • D.\(n = 3\)

Answer: B

The number of equations is \(n = 6\).

Three free particles in space have \(3 \times 3 = 9\) coordinates. The three fixed distances \(m_{1}m_{2}\), \(m_{2}m_{3}\) and \(m_{1}m_{3}\) are three holonomic constraints, leaving

\[ 9 - 3 = 6 \]

degrees of freedom. The system is a rigid body (three non-collinear points fix it), which indeed has 3 translational and 3 rotational degrees of freedom. There is one Euler-Lagrange equation per generalised coordinate, so 6 equations. 9 ignores the constraints, 3 counts only the constraints, and 7 has no basis.

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