Engineering Mechanics
The efficiency of a lifting machine is the ratio of

All of these
Output to the input
Mechanical advantage to the velocity ratio
Work done by the machine to the work done on the machine

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Engineering Mechanics
The static friction

Bears a constant ratio to the normal reaction between the two surfaces
All of these
Always acts in a direction, opposite to that in which the body tends to move
Is independent of the area of contact, between the two surfaces

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