Theory of Machine
Which of the following statement is correct as regard to the difference between a machine and a structure?

All of these
A machine transforms the available energy into some useful work, whereas in a structure no energy is transformed into useful work
The parts of a machine move relative to one another, whereas the members of a structure do not move relative to one another
The links of a machine may transmit both power and motion, whereas the members of a structure transmit forces only

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Theory of Machine
A body is said to be under forced vibrations, when

No external force acts on a body, after giving it an initial displacement
A body vibrates under the influence of external force
None of these
There is a reduction in amplitude after every cycle of vibration

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Theory of Machine
Two heavy rotating masses are connected by shafts of lengths l₁, l₂ and l₃ and the corresponding diameters are d₁, d₂ and d₃. This system is reduced to a torsionally equivalent system having uniform diameter d = d₁ of the shaft. The equivalent length of the shaft is

(l₁ + l₂ + l₃)/3
l = l₁ + l₂.(d₁/d₂)³ + l₂.(d₁/d₃)³
l = l₁ + l₂.(d₁/d₂)⁴ + l₃.(d₁/d₃)⁴
l₁ + l₂ + l₃

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