问题 选择题

下列对牛顿第二定律表达式F=ma及其变形公式的理解,正确的是(  )

A.由F=ma可知,物体所受的合外力与物体的质量成正比,与物体的加速度成正比

B.由m=

F
a
可知,物体的质量与其所受合外力成正比,与其运动加速度成反比

C.由a=

F
m
可知,物体的加速度与其所受合外力成正比,与其质量成反比

D.由m=

F
a可知,物体的质量决定于它的加速度和它受到的合外力

答案

A、物体的合外力与物体的质量和加速度无关.故A错误.

    B、物体的质量与合外力以及加速度无关,由本身的性质决定.故B、D错误.

    C、根据牛顿第二定律a=

F
m
可知,物体的加速度与其所受合外力成正比,与其质量成反比.故C正确.

故选C.

阅读理解

阅读理解。

     Fear is an emotion like others such as happiness, anger, hurt, sadness. We need emotions to process

information we receive and decide how to respond. Being afraid of fast cars, for example, is something

that might protect us from harm Being afraid of the consequence of a choice may prevent us getting into

trouble.

     Fears in young children commonly center on certain animals like snakes or big dogs. Fears are caused

often because of experiences or ideas expressed by others, and at times, the media. Many normal fears

during the early years, like men with beards, or large dogs, disappear with age. Those relating to personal

failure and ridicule (嘲笑) remain through adulthood and may need special help to overcome.

     Children's fears are often small, but that doesn't mean they should be dismissed as trivial (微不足道的).

They need to be recognized and accepted as real for that child. Only when we help children understand their

fears can they grow normally in their ability to settle them.

     Research shows that as a child grows up, the center of his fears changes a lot. Things like divorce, a

teacher who "shouted at me", people with guns, bullies, big boys, or "making fun of me" top the list of

childhood fears.

     We cannot always prevent these experiences from happening, but it is essential that children be allowed

to freely express their emotions without judgment. Sympathy and a caring listener will help ease pain of these

fears. Read books and stories to your child about children who have experienced similar fears. This helps

children talk about their fears and find ways to cope. With all emotions, fears become less of a problem for

children as they gain self-confidence and they find that fear is normal and can be dealt with.

1. According to Paragraph 2, we can know that fears in young children _____. [ ]

A. only center on frightening animals

B. are not born but learned

C. can make sure of their safety

D. need special help to deal with

2. Children's fears are often ignored because _____. [ ]

A. they exist widely in the world

B. they will disappear with age

C. people thought they are not worth considering

D. they are often imaginary

3. In dealing with children's fears, the final goal of the parents is to _____. [ ]

A. stop unpleasant experiences happening to children

B. let children know about the nature of fear

C. create an environment where there is nothing to fear

D. develop children's ability to treat their fears on their own

4. From the passage we can learn that _____. [ ]

A. some childhood fears may have a lasting influence

B. children should avoid the media to reduce their fear

C. fears can be judged according to the environment

D. children with similar fears can communicate easily

5. What's the best title for this passage? [ ]

A. Children's fears.

B. How to overcome children's fears.

C. The kind of children's fears.

D. The bad effects of fears.

单项选择题