解题思路:
(1)要求施力方向向下,从绳子末端开始,依次绕过定滑轮、动滑轮,这样反复完成,绕法如图所示;
(2)
η
=
×100%
=
×100%
=
×100%
=
80
%
;
(3)挂
800
N
重物时,绳重及摩擦不计,
∵
F
1
=
(
G
动
+
G
物
)
,
∴
G
动
=
2
F
1
−
G
物
=
2
×
500
N
−
800
N
=
200
N
挂
600
N
重物时,拉力:
F
2
=
(
G
动
+
G
物
)
=
(600
N
+
200
N
)
=
400
N
,
s
=
2
h
=
2
×
5
m
=
10
m
,
拉力做功:
W
=
F
s
=
400
N
×
10
m
=
4000
J
,
(1)如图所示 (2)80% (3)4000J
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