解:
(1 )∵h=2m ,t=4s ,
∴V=====0.5 m/s.
(2 )∵m=50kg ,g=9.8N/kg ,
∴G=mg=50kg ×9.8N/kg=490N ,
又∵h=2m ,
∴W 有用=Gh=490N ×2m=980J .
(3 )∵S=3h=3 ×2m=6m ,F=200N ,
∴W总=FS=200N ×6m=1200J .
(4 )∵W有用=980J ,W总=1200J ,
∴η=×100%=×100%=81.7% .
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