W有用=Gh=16N×2m=32J，W总=Fs=10N×4m=40J，η= W有用 W总 = 32J 40J =80%=0.8；∵W总=W有用+W额，∴W额=W总-W有用=40J-32J=8J，∵W额=fs，∴f= W额 s = 8J 4m =2N．故选B．
