P. 3994. A 60 kg boy notices a nice fruit on a tree. Even with stretched arms he cannot reach it, he is a 0.5 m below the fruit. The boy crouches down - his centre of mass descends 0.4 m - and then he leaps up. Thus he just reaches the fruit. Assume that he exerts a constant force on the ground while he touches it.
a) What is this force at least?
b) What is the average power of the boy?
P. 3998. A cylinder with a piston in it encloses some air. Initially the volume of the air is V=0.1 m3, its temperature is 27 oC, and its pressure is p=113 kPa. The system is operated as a heat engine, while the air is taken through a cyclic process which consists of two isobaric processes at pressures of p and 2p and two isochoric processes at volumes of V and 3V.
a) What is the work done by the gas in the cyclic process?
b) What is the efficiency of this heat engine?