如图甲所示，两个平行金属板P、Q正对竖直放置，两板间加上如图乙所示的交变电压。t=0时，Q板比P板电势高U0，在两板的正中央M点有一电子在电场力作用下由静止开始运动（电子所受重力可忽略不计），已知电子在0~4t0时间内未与两板相碰。则电子速度方向向左且速度大小逐渐减小的时间是( )
A．0<t<t0
B．t0 <t<2t0
C．2t0 <t<3t0
D．3t0 <t<4t0
答案：D
题目分析：开始时Q板电势比P板高，电场线向左，负电荷受力向右，因此在0<t<t0内，电子向右加速运动；A错误；t0时刻电场反向，电子开始向右减速运动，到2t0时刻，速度恰好减小到零，B错误；在2t0 <t<3t0时间内，电子向左加速运动，C错误；在3t0时刻，电场再次反向，电子开始向左减速运动，到4t0时刻，速度刚好减小到零，D正确。
Some scholars have associated trade unions with the medieval craft guilds (中世纪的行会), but there are important differences between the two. The guild members were masters craftsmen who owned capital and often employed workers. Unions are known as associations of workers with similar skills. 2. In the past, individual worker had no control over the conditions of their working lives; political and economic power was concentrated in the hands of wealthy business owners. Workers found, however, that there was strength in uniting.From their earliest years, union objectives have been higher wages and improved working conditions. 3. Employers resisted, of course. They made great efforts to stop union organizing its activities. Union members were fired, workers were forced to sign contracts in which they promised not to join a union, and companies hired strike－breakers (罢工破坏者) and even gunmen to frighten organizers. 4. One of the earliest successful labour organizations in the United States was the Knights of Labour, founded in 1869. The Knights, which included both skilled and unskilled workers, attempted to organize all workers into one great union. After it successfully struck the Wabash railroad owned by Jay Gould in 1885, its popularity and power grew dramatically.In 1886 th  e Knights had 700,000 members.5. The decline of the Knights of Labour, however, came quickly. The strike against Gould was gradually broken, and the Knights radical positions on social issues cost them public support. In the end, a lack of unity as well as the rapid inflow of unskilled immigrants weakened the union's economic power, and the organization came to an end. |