The room-temperature stress $$\left( \sigma \right)$$ - strain $$\left( \varepsilon \right)$$ curves of four materials $$P, Q, R$$ and $$S$$ are shown in the figure below. The material that behaves as a rigid perfectly plastic material is

In an off-set slider crank mechanism, shown in figure, the crank is rotated at a constant speed of $$150$$ rpm. The value of the angle $$\theta $$ shown in the figure is $${20^ \circ }$$ . What is the ratio of forward to return stroke time? Can this mechanism be used in an application involving quick return?

An ideal gas of mass $$m$$ is contained in a rigid tank of volume $$V$$ at a pressure $$P.$$ During a reversible process its pressure reduces to $${P_1}.$$ Following statements are made regarding the process.
$$(P)$$ Heat is transferred from the gas.
$$(Q)$$ Work done by the gas is zero.
$$(R)$$ Entropy of the gas remains constant.
$$(S)$$ Entropy of the gas decreases.

Among the above statements, the correct ones are