问题 问答题

真空中存在空间范围足够大的、水平向右的匀强电场.在电场中,若将一个质量为m、带正电的小球由静止释放,运动中小球的速度与竖直方向夹角为37°(取sin37°=0.6,cos37°=0.8).现将该小球从电场中某点以初速度v0竖直向上抛出.求运动过程中:

图16-2-5

(1)小球受到的电场力的大小及方向;

(2)小球从抛出点至最高点的电势能变化量;

(3)小球的最小动量的大小及方向.

答案

(1) 水平向右 (2) (3) 方向与电场方向夹角为37°斜向上

根据题设条件,电场力大小

电场力的方向水平向右.

(2)小球沿竖直方向做匀减速运动,速度为vy

沿水平方向做初速度为0的匀加速运动,加速度为ax

小球上升到最高点的时间,此过程小球沿电场方向位移

电场力做功

故小球上升到最高点的过程中,电势能减少.

(3)水平速度vx=axt,竖直速度vy=v0-gt

小球的速度

由以上各式得出

解得当时,v有最小值

此时,,, ,

即与电场方向夹角为37°,斜向上

小球动量的最小值为

.

最小动量的方向与电场方向夹角为37°,斜向上.

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阅读下面短文并回答问题,然后将答案写到答题卡相应的位置(请注意问题后的词数要求)。

【1】Who says you need super power to save the world? On April 22, more than a billion people worldwide celebrated Earth Day by taking part in green activities in their communities. However, you needn’t wait until Earth Day. You needn’t be an adult, either! Every day, young people across the globe are doing their part to keep the Earth healthy and green. These are just a few kid heroes for the planet.

Manatee (海牛) minder

【2】When she was in second grade, Stephanie Cohen read about a baby manatee hit by a boat off the coast of Florida. Biologists saved the manatee. But caring for the sea creature was expensive. Stephanie took up a collection and raised 27 dollars in one day. Now 18, Stephanie still raises money to help these "Gentle Giants".

Goodbye paper bags

【3】When Adrienne Smith, 15, was in sixth grade, she noticed that many of her classmates carried paper lunch bags. To help kid cut down on waste, she invented an eco-friendly lunch bag. It is partly made from recycled plastic bottles. "Kids like it because they can fold it up and put it in their backpacks," said Adrienne. She sells the bags at greenearbag.com. And some of the money goes to charity.

He's skiing to save the earth

【4】Parker Liantaud, 15, wanted to raise awareness about the effects of climate change on the polar regions. He set out to become one of the youngest people ever to ski the North Pole. Although the cruel conditions forced Parker and his guide to finish the last 15 miles of the journey by helicopter, Parker felt he met his goal to inspire others. "All young people have the power to make a change," he said.

He ______ the earth

【5】Every year, Max Kesselman, from Pennsylvania, USA, celebrates his birthday December 3 by cleaning up the Earth. When Max was 6, he noticed trash on the sides of the road. He wrote a letter to local leaders suggesting a cleanup day on his birthday.(words:353)

小题1:What is the main idea of the passage? (no more than 8 words)

小题2:What does the word "it" (Line 3, Paragraph3) probably refer to? (no more than 5 words )

小题3:Fill in the blanks of the title of Paragraph 5 with proper words. (no more than 5 words)

小题4:Why didn't Parker ski the North Pole all the way?(No more than 7 words)

小题5:Write out  at least one other Green Activity you can take part in?(no more than 8 words)