1、2题括号内是解释.
1.(机械效率=W有用/W总=Gh÷FS绳=75% ) G=机械效率×W有用(FS绳)÷h=75%×3.6N ×4m(S绳=nh=4×1=4m)÷1m=10.8N
2.W额外=W总-W有用=FS绳(S绳=nh=4m)-Gh=3.6N×4m-10.8N×1m=3.6J
3.物体提高还是1米?
F拉=1/nG=1/4×3N=3/4N
S绳=nh=4×1m=4m
机械效率=W有用/W总=Gh/FS绳=(3N×1m)÷(3/4N×4m)=100%
第三题条件不够可能答的有错,见谅.