匿名
匿名 發問於 科學及數學其他 - 科學 · 7 年 前

係靜力平衡地問題

該....怎麼解 這題..靜力平衡 ??

http://a.imageshack.us/img89/3205/fvc0.jpg

更新:

題目有說:

最大壓縮力放在最先,最大張力放在最後

所以關於(b)的答案,我認為是,

依題目要來排列為,

3(最大壓縮力),2,1(對於B點來說力為零),4,5,6(對於B點來說,有最大的張力)

我這樣的解答 對嗎? 還是哪裡有錯呢?

謝謝指教:)

2 個解答

評分
  • 天同
    Lv 7
    7 年 前
    最佳解答

    (a) The compressive force (壓縮力)on A is maximum and is equal to the weight of the box when the box is placed directly above A.

    The force on A is zero when the box is placed directly above B (position 3), as there is no moment (力矩) produced about B.

    When the box is to the left of B, it produces a anticlockwise moment about B. Hence, such moment must be balanced by a clockwise moment given by A. That is to say, the force acting on the plank is upward. Then by action-reaction law (Newton's Third Law), A is under a downward compression force.

    When the box is placed to the right of B, by taking moment about B, the box gives a clockwise moment. Hence, there must be a downward force given by A on the plank such as to produce a anticlockwise moment about B to balance the clockwise moment given by the box. That is, the the force acting on A is upward (because of action and reaction) which indicates a extension force(張力) .

    The arrangement is:

    Compression force: 1 > 2

    Zero force: 3

    Extension force: 6 > 5 > 4

    (b) The force on B is zero when the box is on A (position 1)

    By taking moment about A, the box gives a clocwise moment on other positions. Hence, the force acting on the plank given by B must be upward so as to give an anticlockwise moment about A. That is, B is under a compression force. This force is maximum when the box is on B (position 3).

    The arragement is,

    Zero force: 1

    compression force: 3 > 6 > 5 > 4=2

    The longer the distance of the box from A, the larger is the moment, hence the greater is the compression force on B.

     

    2013-06-20 16:10:52 補充:

    Sorry, I made a mistake in part (b)

    Taking moment about A, W.x = R.L

    where W is the weight of the box at distance x from A

    R is the upward normal force on B, L is the distance between AB

    2013-06-20 16:15:25 補充:

    cont'd...

    Then, R = Wx/L the larger the value of x, the greater the force R

    The arrangement is:

    6 > 5 > 4 > 3(= W) > 2 > 1(=0)

    All of the forces are compression. At positions 4,5 and 6, the force is gretaer than W. At position 2, the force is smaller than W.

    2013-06-20 16:16:45 補充:

    因字數所限,在[意見]欄繼續

  • 匿名
    7 年 前

    題目有說:

    最大壓縮力放在最先,最大張力放在最後

    所以關於(b)的答案,我認為是,

    依題目要來排列為,

    3(最大壓縮力),2,1(對於B點來說力為零),4,5,6(對於B點來說,有最大的張力)

    我這樣的解答 對嗎? 還是哪裡有錯呢?

    謝謝指教:)

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