Theory of Machine
The essential condition of placing the two masses, so that the system becomes dynamically equivalent, is (where l₁ and l₂ = Distance of two masses from the centre of gravity of the body, and kG = Radius of gyration of the body)

l₁l₂ = kG
l₁l₂ = kG²
l₂ = kG
l₁ = kG

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Theory of Machine
A rigid body, under the action of external forces, can be replaced by two masses placed at a fixed distance apart. The two masses form an equivalent dynamical system, if

The sum of mass moment of inertia of the masses about their center of gravity is equal to the mass moment of inertia of the body
The sum of the two masses is equal to the total mass of body
All of these
The center of gravity of the two masses coincides with that of the body

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