题干

如图所示,四个相同的小球A、B、C、D,其中A、B、C位于同一高度h处,A做自由落体运动,B沿光滑斜面由静止滑下,C做平抛运动,D从地面开始做斜抛运动,其运动的最大高度也为h.在每个小球落地的瞬间,其重力的功率分别为PA、PB、PC、PD.下列关系式正确的是(   )

A:PA=PB=PC=PD

B:PA=PC>PB=PD

C:PA=PC=PD>PB

D:PA>PC=PD>PB

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答案(点此获取答案解析)

C

同类题3

阅读下面短文,从短文后各题所给的四个选项()A、B、C和D)中,选出可以填入空白处的最佳选项。

    I sat down at my desk and tried to focus on my studies. In just seven more days, finals would be over and all the students in my college 1 would be leaving to   go home-2 me. I would be moving 3 to another dormitory for the Christmas break.

    I had no 4. I was a student from Pennsylvania attending college in Utah and my family of ten couldn't 5 to pay for my ticket home. The only 6 I was even able to attend college was that I had received several scholarships.

    I started at the white snow dancing through the air. This would be my 7 Christmas away from home. I felt so 8 that I could weep. 9 I swallowed down the ache that was rising in my throat, my phone rang suddenly. I reached over and picked it 10.

    “Hello?”

    “Hi, Teddi. This is Elsie.” What was my boss from high school grocery job 11 me for? “All the workers here 12 that you weren't able to make it home this year.13 we bought you a ticket. You're coming home!”

    I screamed.

    Fifteen minutes later I was off the14and jumping up and down for 15. Neighbors came over to see what the 16 was all about.

    “I'm going home for Christmas! My17 at my high school job bought me a ticket!” I told them.

    Christmas 18, I was awoken by laughs and screams of joy. The smell of roasting turkey and the wrapping paper(包装纸)that grew higher and higher filled me with love and 19. The kindness of some special fiends 20 me to have the best Christmas ever.

同类题5

阅读理解

    October 15th is the Global Hand washing Day. Activities are planned in more than 20 countries to get millions of people in the developing world to wash their hands with soap(肥皂). For example, donators(捐献者)will give 150,000 bars of soap to schools in Ethiopia.

    Experts say people around the world wash their hands every day, but very few use soap at so-called important moments. These include after using the toilet, after cleaning a baby and before touching food.

    Global Hand washing Day is the idea of the Public-Private Partnership for Hand washing with Soap. Partners include the United Nations Children's Fund, American government agencies, the World Bank and soap makers Unlever and Procter and Gamble. The organizers say all soaps are equally effective at removing disease-causing germ(细菌). They say the correct way to wash is to wet your hands with a small amount of water and cover them with soap. Rub it into all areas, including under the fingernails. Rub for at least twenty seconds. Then rinse well under running water. Finally, dry your hands with a clean cloth or wave them in the air.

    The Partnership for Hand washing says soap is important because it increases the time that people spend in washing hands. Soap also helps to break up the dirt that holds most of the germs. And it usually leaves a pleasant smell. The Partnership for Hand washing also says washing with soap before eating or after using the toilet could save more lives than any vaccine(疫苗)or medicine. Hand washing could also prevent the spread of other diseases. When people get germs on their hands, they can infect(传染)themselves by touching their eyes, noses or mouths. Then they can infect others.