题干

蹦极”是一项刺激的极限运动,运动员将一端固定的长弹性绳绑在踝关节处,从几十米高处跳下。在某次蹦极中,弹性绳弹力F的大小随时间t的变化图象如图所示,其中t2、t4时刻图线的斜率最大。将蹦极过程近似为在竖直方向的运动,弹性绳中弹力与伸长量的关系遵循胡克定律,空气阻力不计。下列说法正确的是

A:t1~t2时间内运动员处于超重状态

B:t2~t4时间内运动员的机械能先减少后增大

C:t3时刻运动员的加速度为零

D:t4时刻运动员具有向下的最大速度

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答案(点此获取答案解析)

B

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    When something great happens to a friend, a family member or a classmate, congratulations are in order! However, some people don't know how to congratulate others. Here are some ways to show your congratulations.

Say “congratulations” with a smile. Remember to express your congratulations with a winning smile. The expression on your face is pretty important. If you say “congratulations” with a serious look on your face, the person will probably think you aren't actually all that happy for him or her.

    Send an en email or a card. Besides saying “congratulations in person, you might want to send a congratulatory note. You can send a simple email or write a message on a card. You can buy some beautiful cards in the store.

    Send a gift. If the occasion is special enough, you can send a gift. You don't need to buy something expensive. Try to buy a suitable gift. All in all, choose a gift that lets the person know you want to celebrate with him or her.

    Spread the word. You can help spread the person's joy by telling other people the good news, so they can congratulate the person as well. If the person prefers to keep the good news private, don't spread the word.

Title:  How to ____

Say “congratulations” with a smile.

★ Express your congratulations with a winning smile.

★____ congratulations with a serous look on your face.

Send an en email or a card.

You can send a simple email or ____ on a card.

Send a gift.

You can buy a suitable gift instead of something expensive.

Spread the word.

★You can tell other people the good news.

★You can spread the good news around if the person ____keep it private.