=+where  is a given length. A linear spring with stiffness k is attached to the mass and fixed to the point (0, 0). The spring has a zero natural length. The gravity acts in the direction −y.

Prove that a system of N particles that are rigidly linked to one another (thus forming a rigid body in space) can be described by 6 degrees of freedom when N > 2.

Analytical Dynamics of Discrete Systems 47 x

y o

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g k