题干

如图所示,倾斜的传送带保持静止,一木块从顶端以一定的初速度匀加速下滑到底端.如果让传送带沿图中虚线箭头所示的方向匀速运动,同样的木块从顶端以同样的初速度下滑到底端的过程中,与传送带保持静止时相比(   )

A:木块在滑到底端的过程中,摩擦力的冲量变大

B:木块在滑到底端的过程中,摩擦力的冲量不变

C:木块在滑到底端的过程中,木块克服摩擦力所做功变大

D:木块在滑到底端的过程中,系统产生的内能数值不变

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答案(点此获取答案解析)

B

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A report by UNEP联合国环境规划署(United Nations Environment Programme) says that by 2050, the world's population is expected to reach 9.6 billion and feeding us all will require at least 60% more food than we have ever produced before. What's more, if we can't find out a way to do with it, food shortages (短缺) could be a big problem.

A main purpose of the report is to make people be aware of the importance of this challenge. In the past, food production increase relied on the practices like cutting down trees, using chemicals and wasting water. Today we understand the environmental price of those techniques.

“The question now is how to produce much more food and at the same time avoid all the environmental problems that we had in the past,” Searchinger, a famous scientist said.

We need to develop new ways to produce food. We can no longer cut down trees. Instead, we'll have to protect water and land, and reduce air pollution.

The report also mentions the importance of reducing food loss and wasteAccording to the United Nations, the waste adds up to one-third of all the food we produce —an amount that could feed an additional 2 billion people.

The report also suggests that we make changes in our diet. “You have to eat more efficiently (有 效地),” said Searchinger, “and above all that means eating less meat, and especially reducing the growth of beef.”That's because raising cows is an inefficient way of producing food. Of all the energy that goes onto feeding cows, only about 1% is converted (转换) into food for people.