Velocity Ratio (VR): It is ratio of distance moved per unit time by effort to the distance moved per unit time by load.
, where y is distance moved by the effort (P), and x is distance moved by the load (W).
Mechanical Advantage (MA): It is ratio of load lifted and effort applied on a lifting machine. Its significance is that how many times of effort, the load is lifted.
, where W is load lifted in Newton, and P is effort applied in Newton.
Efficiency of Lifting Machine, η: Efficiency of lifting machine is the ratio of work done by the machine to lift the load and work done on the machine. It is simply ratio of output and input.
Ideal Machine: It is a hypothetical machine that has 100% efficiency which is practically not possible. It is also known as frictionless machine or perfect machine.
13.2.1 Frictional Loss in Lifting Machine and Law of Machine
Frictional loss is the part of the inputs or work done by the effort used to overcome the friction of the machine. No machine can be 100% frictionless; therefore, some parts of the inputs are always used to overcome the friction of the machine. Thus, the efficiency of the real machine always lies below 100%.
Frictional loss = Input – Output
The relationship between the load and effort is shown in the graph in Figure 13.2.
Figure 13.2 Relationship Between Load and Effort (Law of Machine)
OA – Effort required overcoming the friction.
where Pi is the ideal effort required to raise a given load, Pa is actual effort required to raise the load, and Pf is effort required to overcome the friction.
For an ideal machine, η = 1
Putting the value of Pi in Eq. (13.1).
The relation between load and effort is given by an equation, P = m·W + C which is known as law of machine. Where m is slope of the line and equal to 
and C is constant known as frictional constant.
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