Velocity ratio (or speed ratio) (V.R.)

Velocity ratio (or speed ratio) (V.R.)

In the pulley systems we have already considered where the mechanical advantage is greater than 1, it might appear at first sight that we are getting more out of the machine than we are putting into it. But while in such cases the load is greater than the effort, it must be remembered that the effort moves through a much greater distance than that of the load. Consequently, the work obtained from the machine is equal to the work put into it, less any work wasted in the machine, but this is discussed more fully in the next section.
From Fig. 8.6 it will be clear that, in order to raise the load by 1 m, each string supporting the load must be shortened by I m. The effort must therefore be applied through a total distance of 4 m. The ratio of the distance moved by the effort to the distance moved by the load in the same time is called the velocity ratio of the machine, i.e.

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