人们根据建筑物如何处理由重力产生的向下的力来分析它们。
Structures can be analyzed in terms of how they deal with downward forces created by gravity.
所以也有向下的力，图钉上的力mg向上。
Mg so also here we have now mg down, so we have at the pin mg up.
向上的力与向下的力相等，所以气球就保持在这一高度。
The force upward equals the force downward so that the balloon stays at the level.
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