题干

如图所示,楔形木块abc固定在水平面上,粗糙斜面ab和光滑斜面bc与水平面的夹角相同,顶角b处安装一定滑轮.质量分别为M、m(M>m)的滑块,通过不可伸长的轻绳跨过定滑轮连接,轻绳与斜面平行.两滑块由静止释放后,沿斜面做匀加速运动.若不计滑轮的质量和摩擦,在两滑块沿斜面运动的过程中(  )

A:两滑块组成系统的机械能守恒

B:重力对M做的功等于M动能的增加

C:轻绳对m做的功等于m机械能的增加

D:两滑块组成系统的机械能损失等于M克服摩擦力做的功

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C,D

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    The uninvolved dad, turning up his nose at diapering(换尿布) and too busy to bathe, dress and play with his kids, is mostly a myth(神话) , a big government survey suggests. Most American fathers say they are heavily involved in hands-on parenting, the researchers found.

    The results are encouraging and important " because others have found the more involved dads  are, the better the outcomes for their children. " said researcher Jo Jones of the National Center for Health Statistics, part of the Centers for Disease Control Prevention. She co-authored the report released Friday.

    "Times have changed," said Robert Loftus, 34, of Yonkers, NY. He quit a six-figure sales job a year ago to care for his two young children while his wife works full time. "We are trying to rethink our priorities (优先考虑的事) and family seem to be NO.1 priority while in the past maybe people were more focused on career. "

    The study involved nearly 4.000 fathers who were interviewed in person between 2009 and 2013.

◇ Key findings among fathers living with children younger than 5 :

*9 in 10 bathed, diapered, helped them use the toilet or get dressed at least several times weekly .

* Even higher numbers played with them and ate meals with them that often

* Almost 2 0ut of 3 read to them at least several times weekly.

◇Among dads living with kids aged 5-18.

* More than 9 0ut of 10 ate meals with them at least several times weekly and talked with them about what happened during the kids' day that often.

* Almost 2 0ut of 3 helped with homework several times weekly.

* About half took their kids to or from activities that often.

    Dr. David Hill, a Wilmington, N. C. pediatrician (儿科医生) said the survey reflects what he's seen among his patients' fathers. Increasingly, fathers rather than mothers take their kids to the doctor. Some "are anxious about changing a diaper, " he said.

    Census(调查) numbers show that there were almost 190,000 stay-at-home dads nationwide last year versus 93,000 in 2010. Loftus, the New York stay-at-home dad, said, "I feel fortunate to be able to be such a hands-on father.  I'm doing the most significant occupation in the world. "