归纳总结、分析比较是一种学习物理的良好习惯．黄杨同学发现近期的物理实验经常使用如图所示的两个电路图，他对此做了如下总结：

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    A wealthy man asked an old scholar to tell his son away from his bad habits. The scholar took the young man for a walk through a garden. Stopping suddenly, he told the boy to pull out a tiny plant growing there.

    The young man made it easily. The old man then asked him to pull out a little bigger plant. The young man pulled hard and the plant came out with its roots. “Now pull out that one,” said the old man, pointing to a little tree. The boy had to use all his strength to pull it out.

    “Now take this one out,” said the old man, pointing at a big orange tree. The young man held the trunk (树干) and tried to pull it out. But it would not move. “It's impossible”, said the boy, panting (喘息) with the effort.

    “So it is with bad habits,” said the old man. “When they are young, it is easy to pull them out. But when they go deeply, they cannot be uprooted.”

    The conversation with the old man changed the boy's life.

如图物体随水平向右运动传送带一起匀速移动，作出物体所受力的示意图【不计空气阻力】．

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新风小学原来参加科技、书法和体育组的学生共有70人，后业又有2人参加科技组，这时参加科技、书法和体育组人数的比是3：2：4．现在三个组各有多少人？