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如图所示，用细线通过轻网兜把质量为m的足球挂在光滑墙壁上（细线延长线通过足球球心）．已知悬线与墙壁的夹角为θ，重力加速度为g，求：

（1）细线的拉力F的大小；

（2）足球对墙壁弹力FN的大小和方向．

答案

对足球受力分析如图所示，则由三力平衡条件可得：

细线的拉力  F=

mg
cosθ

墙壁对球的弹力    FN=mgtanθ，方向水平向左．

由牛顿第三定律可得球对墙壁的压力：F压=FN=mgtanθ，方向水平向右．

答：（1）细线的拉力F的大小为

mg
cosθ
；

（2）足球对墙壁弹力FN的大小为mgtanθ，方向水平向右．

单项选择题

固定污染源排气中二氧化硫的碘量法测定中，为防止二氧化硫被冷凝水吸收，而使测定结果偏低，采样时采样管应加热至（）℃。

A.100

B.120

C.150
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I am honored to be here today, (1) Harvard at this celebration of the College Board’s half century of working to promote (2) and equal opportunity in higher education.
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