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    ---  ___  do you like dolphins?
    ---Because they are very smart.
    A．WhatB．WhenC．WhyD．How

    质量为60kg的小明站在水平地面上，利用如图所示装置提升物体A。物体A的质量mA为72kg，底面积SA为2×10-2m2。当小明施加竖直向下的拉力为F1时，物体A未被拉动，此时物体A对地面的压强p为5×103Pa，小明对地面的压强为p1；当小明施加竖直向下的拉力为F2时，物体A恰好匀速上升，小明对地面的压强为p2，且p1：p2="16:" 15。已知B、C、D、E四个滑轮的质量相同，不计绳重和摩擦，g取10N/kg。求：

    （1）当小明施加竖直向下的拉力F1时，物体A对地面的压力FN；
    （2）每个滑轮所受的重力G0；
    （3）当小明施加竖直向下的拉力F2时，整个装置的机械效率η。
    C
    （1）依据公式p=，可知FN=pS=5×103Pa×2×10-2m2="100N "         ---------- 2分
    （2）当小明施加竖直向下的拉力F1时，物体A、滑轮B、滑轮C及小明的受力情况如答图7所示。
     
    TA1=GA- FN′=720N-100N=620N
    F1=
    小明对地面的压强p1=
    当小明施加竖直向下的拉力F2时，物体A、滑轮B、滑轮C及小明的受力情况如答图8所示。
     
    F2=
    小明对地面的压强p2=

    动滑轮受到的重力G0="60N                                     " ---------- 3分
    （3）拉力F2==225N
    设物体A被提升的高度为h，则滑轮C上升的高度hC=2h，
    绳端上升的距离s=2hC=4h
    η=                        ---------- 2分解析:
    由because引导的答语，其问句要用why来询问。略
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