问题 问答题

在绝缘水平面上放置一质量为m=2.0×10-3kg的带电滑块A,电量为q=1.0×10-7C.在A的左边L=1.2m处放置一个不带电的滑块B,质量为M=6.0×10-3kg,滑块B距左边竖直绝缘墙壁s=0.6m,如图所示.在水平面上方空间加一方向水平向左的匀强电场,电场强度为E=4.0×105N/C,A由静止开始向左滑动并与B发生碰撞,设碰撞的过程极短,碰撞后两滑块结合在一起共同运动并与墙壁相碰撞,在与墙壁发生碰撞时没有机械能损失,两滑块始终没有分开,两滑块的体积大小可以忽略不计.已知A、B与地面的动摩擦因数均为μ=0.5。试通过计算,在坐标图中作出滑块A从开始运动到最后静止的速度——时间图象.(取g=10m/s2

答案

请看解析

设A和B相遇时的速度为v1,相碰后共同运动的速度为v.

根据动能定理,对滑块A有:(qE—μmg)L =" m" v12/2 ………2分

解得:v1="6m/s " …………1分

滑块A从开始运动到与B相碰所用的时间为:

      ……………2分

代入数据解得:                         2分

A、B碰撞动量守恒,有:mv1=(M+m)v …………2分

得: ………1分

滑块A与B碰撞后结合在一起,电场力大小仍然为:

F=qE=1.0×10-7C×4.0×105N/C=4.0×10-2N.方向向左 …………1分

两滑块的摩擦力为: f=μ(m+M)g  ……1分

代入数据解得:f = 4.0×10-2N,方向向右

所以,A、B碰撞后一起以速度v向着墙壁作匀速直线运动.A、B碰后到运动到墙壁处所用的时间为:

 ………1分

A、B一起与墙壁碰撞后,两滑块受到的电场力与摩擦力大小不变,方向都向左,所以A、B与墙壁碰撞后一起以速度v向右做减速运动,直至速度减为零,最后静止.所经历的时间设为t3

代入数据解得: ………2分

v——t图象如图,(每对一段给1分)

单项选择题
单项选择题

Who is poor in America This is a hard question to answer. Despite poverty’s messiness, we’ve measured progress against it by a single statistic: the federal poverty line. In 2008, the poverty threshold was $ 21,834 for a four-member family with two children under 18. By 1his measure, we haven’t made much progress. Except for recessions, when the poverty rate can rise to 15 percent, it’s stayed in a narrow range for decades. In 2007—the peak of the last business cycle—the poverty rate was 12.5 percent; one out of eight Americans was "poor. " In 1969, another business-cycle peak, the poverty rate was 12.1 percent. But the apparent lack of progress is misleading for two reasons.

First, it ignores immigration. Many immigrants are poor and low skilled. They add to the poor. From 1989 to 2007, about three quarters of the increase in the poverty population occurred among Hispanics—mostly immigrants, their children, and grandchildren. The poverty rate for blacks fell during this period, though it was still much too high (24.5 percent in 2007). Poverty "experts" don’t dwell on immigration, because it implies that more restrictive policies might reduce U.S. poverty.

Second, the poor’s material well-being has improved. The official poverty measure obscures this by counting only pretax cash income and ignoring other sources of support. These include the earned-income tax credit (a rebate to low-income workers), food stamps, health insurance (Medicaid), and housing subsidies. Although many poor live hand to mouth, they’ve participated in rising living standards. In 2005, 91 percent had microwaves, 79 percent air-conditioning, and 48 percent cell phones.

The existing poverty line could be improved by adding some income sources and subtracting some expenses (example: child care). Unfortunately, the administration’s proposal for a "supplemental poverty measure" in 2011—to complement, not replace, the existing poverty line—goes beyond that. The new poverty number would compound public confusion. It also raises questions about whether the statistic is tailored to favor a political agenda.

The "supplemental measure" ties the poverty threshold to what the poorest third of Americans spend on food, housing, clothing, and utilities. The actual threshold not yet calculated—will probably be higher than today’s poverty line. Moreover, this definition has strange consequences. Suppose that all Americans doubled their income tomorrow, and suppose that their spending on food, clothing, housing, and utilities also doubled. That would seem to signify less poverty—but not by the new poverty measure. It wouldn’t decline, because the poverty threshold would go up as spending went up. Many Americans would find this weird., people get richer, but "poverty" stays stuck.

What produces this outcome is a different view of poverty. The present concept is an absolute one: the poverty threshold reflects the amount estimated to meet basic needs. By contrast, the new measure embraces a relative notion of poverty: people are automatically poor if they’re a given distance from the top, even if their incomes are increasing.

The author thinks the existing poverty line()

A. is a faithful measure of poverty

B. is not adequate as a measure

C. is not as good as the supplemental measure

D. should have been discarded long ago