Tom is from London. He's now in Beijing with his parents. His father and mother teach in a school. He studies in a middle school. The school is four miles from his home. He usually walks to school. He says, "It can make me healthy." Sometimes when it rains, he takes a bus. He's never late, and he's always the first one to get to school. He's good at all his subjects, and often helps us with our English. We all love him very much.

【健康成长录】

镜头一：五一假期，牧城公园各种花开得正艳，游人纷纷挤进花圃竞相合影留念，照相机闪烁不停。此时，一个六七岁模样的小女孩，默默地蹲在拍照的众人身后，用稚嫩的小手扶起半倒在地的花枝。

镜头二：学生小葛捡到200元钱，在寻找失主的过程中，却迎来了4位“失主”——阿姨A：“这是我在x x路上丢的钱”，结果路名不对；——阿姨B：“这200元是我的买鞋钱”，经核实，阿姨脸红；——叔叔A：“谢谢好孩子，拾金不昧”，一盘问，情况不符；——叔叔B：“好孩子，这钱分你一半”，小葛无语，陷入沉思……

结合以上镜头，思考下列问题：

正结黄瓜的植株，其吸收的水分主要用于(   )

重力为600牛的某同学躺在床上时，他对床的压力为____ 牛；若此时他对床面的压强为3000帕，其物理意义表示床面每平方米面积上所受____ ，这时他与床面的接触面积为____ 米²．