Theory of Machine
The two elements of a pair are said to form a higher pair, when they

Have a line or point contact when in motion
Have a surface contact when in motion
Permit relative motion
Are kept in contact by the action of external forces, when in motion

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Theory of Machine
A thin circular disc is rolling with a uniform linear speed, along a straight path on a plane surface. Which of the following statement is correct in this regard?

The centre of the disc has centrifugal acceleration
All points of the disc have the same velocity
The point on the disc making contact with the plane surface has zero acceleration
The centre of the disc has zero acceleration

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