Theory of Machine
When the two elements of a pair have a surface contact when relative motion takes place and the surface of one element slides over the surface of the other, the pair formed is known as a

Self-closed pair
Force-closed pair
Lower pair
Higher pair

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Theory of Machine
Higher pairs are those which have

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

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Theory of Machine
The static balancing is satisfactory for low speed rotors but with increasing speeds, dynamic balancing becomes necessary. This is because, the

Unbalanced couples are caused only at higher speeds
Effects of unbalances are proportional to the square of the speed
Effects of unbalances are directly proportional to the speed
Unbalanced forces are not dangerous at higher speeds

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