人的质量为M，通过定滑轮将一个质量为m的物体从高处放下。物体以a(a＜g）加速下降。则地面对人的支持力为 (  )

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='17783' and id < 1503023 and id<>'1503023' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='17783' and id > 1503023 and id<>'1503023' order by id asc limit 5