问题 选择题

关于匀速圆周运动的向心加速度,下列说法正确的是 (     )       

A.向心加速度是描述线速度变化的物理量

B.向心加速度只改变线速度的方向,不改变线速度的大小

C.向心加速度恒定

D.向心加速度的方向时刻发生变化

答案

BD

题目分析:向心加速度是描述线速度方向变化快慢的物理量,A错;只改变线速度的方向,不改变线速度的大小,B对;加速度方向指向圆心,时刻改变,C错,D对,所以本题选择BD。

完形填空

阅读下面短文,简要回答所给问题。

To Protect the environment, China has put a ban on thin plastic bag (plastic bag thinner than 0.025mm) From June 1, 2008. China stopped the production, sale and use of these plastic bags to the public.

Plastic bags can cause “white pollution”, because they can take 400-1000 years to break down. They pollute the soil and water. Plastic bags also cause a serious danger to animals. Animals often mistake plastic bags for food. Thousands of them die each year after eating plastic bags.

So what kind of bags will be the best to use when we are shopping? Recently some students made their own shopping bags. Most students used old clothes to make their bags. They painted pictures like the Olympic rings on them. But Li Lei had a more traditional way to make her bag. She weaved(编织) a bamboo basket . It was not an easy job. With her grandfather’s help, Li Lei spent the whole afternoon making it .What can we do? Refuse plastic bags and persuade people to use environmentally shopping bags instead of plastic ones. Try to recycle the one we have .

小题1:Did Li Lei make her shopping bag by herself?

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小题2:Why do plastic bags cause a serious danger to animals?

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小题3:What kind of material did most students use to make their shopping bags?

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小题4:When did China stop the production , sale and use of these plastic bags ?

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小题5:According to the passage, what should we do to protect the environment? (One example is OK.)

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单项选择题