问题 多选题

质量均为m的物块A和物块B相互接触放在水平面上,如图所示.若对A施加水平推力F,则两物块沿水平方向做加速运动.关于A对B的作用力,下列说法正确的是(  )

A.若水平面光滑,物块A对B的作用力大小为F

B.若水平面光滑,将推力F增大1N,则物块A对B的作用力也增大1N

C.若物块A与地面、B与地面的动摩擦因数均为μ,则物块A对B的作用力大小为

F
2

D.若物块A与地面的动摩擦因数为μ,B与地面的动摩擦因数为2μ,则物块物块A对B的作用力大小为

(F+μmg)
2

答案

A、若水平面光滑,则对整体受力分析可知;F=(m+m)a,解得a=

F
2m

再对B分析,B水平方向只受A的作用力,由牛顿第二定律可得:T=ma=

F
2
,故AB错误;

C、若AB和地面动摩擦因数相同,对整体分析有:F-μ2mg=2ma′;则B受力为T′-μmg=ma′,解得:T′=μmg+

F
2
-μmg,故C正确;

D正确;

D、对整体分析有:F-μmg-2μmg=2ma″;再对B分析则有:T″-2μmg=ma″,解得:T″=

F+μmg
2
;故D正确;

故选:CD.

单项选择题
单项选择题

Fate has not been kind to the western grey whale. Its numbers have dwindled to 130 or so, leaving it “critically endangered” in the eyes of the International Union for the Conservation of Nature. Fishing-nets, speeding ships, pollution and coastal development threaten the few that remain. Most recently, drilling for oil and gas in their main summer feeding grounds, near Sakhalin island off Russia’s Pacific coast, has brought fresh risks for the luckless creatures. Yet the rush to develop Sakhalin’s offshore fields may yet be the saviour of the species.

When drilling was first discussed in the 1990s, there were muted complaints. When a consortium called Sakhalin. Energy, led by Royal Dutch Shell, announced plans to build an oil platform and lay pipelines in the only bay where the whales were known to congregate, these protests proliferated.

In response, the consortium established an independent panel to advise it on how best to protect the whales and promised to fund its work. It subsequently agreed to change the route of the pipeline at the panel’s suggestion, although it refused to move the platform, as other critics had demanded. It also agreed either to follow the panel’s recommendations in future or to explain publicly why it was rejecting them.

The platforms and pipelines are now complete. Sakhalin Energy exported its first cargo of liquefied natural gas last week. The project, says Shell, is an engineering triumph and a commercial success despite all the controversy.

But has it been a success for the whales Sakhalin Energy says their number seems to be growing by 2.5% a year, although Ian Craig, the firm’s boss, admits that the cause might be greater scrutiny rather than population growth. The scientists on the panel still seem worried. They complain that the firm has not always provided the information they need to assess the threat to the whales. It also has not always followed advice, the scientists’ advice about how noisy construction might scare the animals away, for example, or the speed that boats should travel to minimize the risk of hitting the whales. The scientists warn that the loss of just a few fertile females would be enough to tip the population into irrevocable decline. Last summer, there seemed to be far fewer whales around than normal.

On the other hand, the panel knows this only because Sakhalin Energy funds lots of research on the whales. As a result, it has discovered that they have a wider range than originally thought, which might explain why so few of them showed up off Sakhalin island last year.

Therefore, it is hard to escape the conclusion that, for creatures with a lot as sorry as the western grey whale, a nearby oil project is something of a blessing.

To ease the protest against its projects, Sakhalin Energy()

A. moved its platform from the bay where the whales congregate

B. established a panel to be in charge of its public relationship

C. rejected its program near Sakhalin’s offshore

D. organized some experts to study and protect the whales