问题 多选题

南方某地发生泥石流灾害,一辆挖掘机在抗险救灾过程中,刚好将一大石块抵在竖直崖壁上保持静止,如图所示,则有(  )

A.此时石块受的力可能是三个也可能是四个

B.挖掘机斜向上支撑力越大,崖壁对石块的摩擦力越大

C.挖掘机支撑力适当减小方向不变,可将石块缓缓放下来

D.挖掘机支撑力逐渐增大方向不变,一定可使石块沿崖壁向上运动

答案

A、对石块受力分析可知,石块一定受到重力、崖壁的支持力N、挖掘机的支撑力F,如图,当Fsinθ=mg时,石块不受静摩擦力,当Fsinθ≠mg时,石块才受静摩擦力.所以此时石块受的力可能是三个也可能是四个.故A正确.

B、若石块的静摩擦力方向向上时,如图,挖掘机斜向上支撑力越大,崖壁对石块的摩擦力越小.故B错误.

C、若挖掘机支撑力适当减小方向不变时,石块的最大静摩擦力减小,石块的重力不变,则可将石块缓缓放下来.故C正确.

D、挖掘机支撑力逐渐增大方向不变,支撑力沿崖壁向上的分力Fsinθ,但石块的最大静摩擦力也增大,不一定能使石块沿崖壁向上运动.故D错误.

故选AC

判断题
阅读理解

Businesses are witnessing a difficult time, which has in turn produced influence on consumers’ desire to go green. However, shoppers are still laying stress on environmental concerns.

Two thirds of customers say that environmental considerations inform their purchases to the same degree as they did a year ago, while more than a quarter say that they are now even better aware of the environmental effect on what they buy.

This may help to influence how shops store goods on their shelves. And the companies should still make efforts to become more environmentally friendly. Two out of three people think it is important to buy from environmentally responsible companies, with about one in seven saying that they had even decided to take their custom elsewhere if they felt a company’s environmental reputation was not good enough.

Harry Morrison, chief executive(主管)of the Carbon Trust, sympathizes:“I understand this situation where survival is very important now. But from environmental considerations, the clock is ticking—we don’t have much time. In addition, cutting carbon has an immediate effect as costs drop and a medium-term benefit for the brand.”

Larger companies have an extra motivation to look at reducing their carbon footprint, as new rules next year will require businesses to buy carbon allowances to make up for their emissions(排放). Those that have taken early action will have a head start. More than two thirds of consumers are not clear about which companies are environmentally responsible. This suggests that firms that are able to relay clearly their message to the public will be in a pole position to attract shoppers.

The Carbon Trust believes that it can help by informing customers about the good work companies are doing. “When companies are granted(授予)the standard, they can use a logo(标识)in all their marketing which makes it clear that they are working towards cutting emissions,” Mr. Morrison said.

小题1: What’s the main idea of the passage?

A.Businesses are finding ways to send their message to the shoppers.

B.Companies will soon get information about cutting carbon emissions.

C.Firms are making efforts to encourage customers to keep goods at home.

D.Firms are urged to cut carbon emissions by shoppers’ environmental awareness.小题2: The underlined word “inform” in Paragraph 2 probably means“      ”.

A.affect

B.change

C.disturb

D.reject小题3:According to Harry Morrison, businesses          .

A.will benefit from cutting carbon emissions

B.should buy carbon allowances for shoppers

C.are required to make up for their carbon emissions

D.have encouraged shoppers to take their custom elsewhere小题4:We can learn from the passage that businesses will        .

A.have a strong desire to reduce costs

B.use the same logo in their marketing

C.gain advantages by taking early action

D.attract more shoppers by storing goods