Theory of Machine
The two links OA and OB are connected by a pin joint at O. If the link OA turns with angular velocity ω₁ rad/s in the clockwise direction and the link OB turns with angular velocity ω₂ rad/s in the anticlockwise direction, then the rubbing velocity at the pin joint O is (where r = Radius of the pin at O)

(ω₁ - ω₂) 2r
ω₁.ω₂.r
(ω₁ + ω₂) r
(ω₁ - ω₂) r

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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

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

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