For the system block △E int block Wrty Ts+w =f+W(=7 IfWr=-fs,△E intblock =o.It is in disagreement with observation So, it must be W,<fs w,=fs is correct only if the object can be treated as a particle(不考虑内部结构时) Ch 13 Energy III: Conservation of energyCh.13 Energy III: Conservation of energy For the system = block Eint,block =WT +Wf = Ts +Wf Wf = fs + (f=T) If Wf =-fs, . It is in disagreement with observation. Eint,block = 0 W fs So, it must be: f  is correct only if the object can be treated as a particle(不考虑内部结构时). W fs f =