Theory of Machine
Lower pairs are those which have

Two elements that permit relative motion
Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion
Elements of pairs not held together mechanically

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

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Theory of Machine
The inversion of a mechanism is

Obtained by reversing the input and output motion
Turning its upside down
Obtained by fixing different links in a kinematic chain
Changing of a higher pair to a lower pair

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