(8分)如图所示装置 ,绳重及摩擦不计,装卸工人将重为800N的货物提至高处,人对绳的拉力F为500N,货物在1min内

1个回答

  • 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

    +

    G

    )

    G

    =

    2

    F

    1

    G

    =

    2

    ×

    500

    N

    800

    N

    =

    200

    N

    600

    N

    重物时,拉力:

    F

    2

    =

    (

    G

    +

    G

    )

    =

    (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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