Reversibility of a Machine
If the removal of effort while lifting results in lowering of the load, the machine is said to be reversible. The machine is said to be self-locking if the load is not lowered on removal of the effort.
A lifting machine is reversible if its efficiency is greater than 50 percent and self locking if its efficiency is less than 50 percent.
This can be proved as follows. Suppose a load W has been lifted to a height h. The potential energy of the load is Wh. Let us say the frictional energy loss is Ef. Now, in order load can come back to its previous position, its potential energy must be more than friction loss. In the limiting case, Wh=Ef. In that case, the efficiency will be 50 %.
Machines
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Law of Machines The relationship between the load lifted and the effort required in a machine is called the law of machine. This is generally a straight line which does not pass through the origin. This is found by conducting experiments in which efforts required for lifting different loads are determined and then load versus effort graph as shown in the following figure is plotted: |