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问题 选择题

木块A、B的重力分别是50N和60N，它们与水平地面之间的动摩擦因数均为0.20，夹在A、B之间的轻弹簧被压缩了2cm，弹簧的劲度系数为400N/m，系统置于水平地面上静止不动．现用F=1.5N的水平拉力作用在木块B上，如图所示，力F作用后（　　）

A．木块A所受摩擦力大小是10N

B．木块A所受摩擦力大小是8N

C．木块B所受摩擦力大小是9.5N

D．木块B所受摩擦力大小是7N

答案

最大静摩擦力可以近似的认为和物体受到的滑动摩擦力的大小相等，

所以A物体受到的滑动摩擦力为f=μFN=0.2×50N=10N，

B物体受到的滑动摩擦力为f′=μFN=0.2×60N=12N，

此时弹簧的弹力的大小为F=kx=400×0.02=8N，

所以弹簧的弹力的大小小于物体受到的最大静摩擦力的大小，物体处于静止状态，

A受到的摩擦力大小是8N，方向向右，B受到的摩擦力大小也是8N，方向向左，

当在B上在施加向右的拉力F后，B受到的摩擦力的大小为9.5N，方向向左，所以BC正确，AD错误．

故选BC．

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单项选择题

In the United States, older people rarely live with their adult children. But in many other cultures children are expected to care (1) their aged parents. In some parts of Italy, the percentage of adult children who (2) with their parents (3) 65 to 70 percent. In Thailand, too, children are expected to take care of their elderly parents; few Thai elderly live (4) . What explains these differences in living arrangements (5) cultures Modernization theory (6) the extended family household to low levels of economic development. In traditional societies, the elderly live with their children in large extended family units for economic reasons. But with modernization, children move to urban areas, leaving old people (7) in (8) rural areas. Yet modernization theory cannot explain why extended family households were never common in the United States or England, or why families in Italy, which is fully modernized, (9) a p tradition of intergenerational living. Clearly, economic development alone cannot explain (10) living arrangements. Another theory associated intergenerational living arrangements with inheritance patterns. In some cultures, the stem family pattern of inheritance (11) . (12) this system, parents live with a married child, usually the oldest son, who then (13) their property when they die. The stem family system was once common in Japan, but changes in inheritance laws, (14) broader social changes brought (15) by industrialization and urbanization, have (16) the (17) .In 1960 about 80 percent of Japanese over 65 lived with their children; by 1990 only 60 percent did-a figure that is still high (18) U.S. standards, but which has been (19) steadily. In Korea, too, traditional living arrangements are (20) : the percentage of aged Koreans who live with a son declined from 77 percent in 1984 to 50 percent just 10 years later. Although most elderly Koreans still expect to live with a son, their adult children do not expect to live with their children when they grow old.

6（）

A.associated

B.linked

C.united

D.combined
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