A personal alarm is designed for old people to ______.
A、detect safety conditions at home
B、avoid falls in the bathroom
C、send out signals for help
D、make phone calls easily
如图所示,导体棒ab、cd放在光滑的水平导轨上,cd棒通过滑轮悬挂一个质量为m的物块,整个装置处于磁感应强度大小为B、方向竖直向下的匀强磁场中,ab在外力作用下以速度v1匀速向右运动时,cd棒由静止释放,设ab、cd的长度均为L,ab棒的电阻为r1,cd棒的电阻为r2,导轨足够长且电阻不计,求: (1)cd棒开始运动的方向与ab棒匀速运动速度v1取值的关系; (2)稳定状态时,cd棒匀速运动的速度; (3)稳定状态时,回路的电功率P电和外力的功率P外.
General Blair had been in the army _____ for 35 years when he retired from the navy.
A、service
B、work
C、job
D、homework
According to the passage, pocket calculators ______.
A、will help students think fast
B、will make math too easy for them
C、will do harm to students’ thinking ability
D、will contribute to the spread of new technology
马克思说:“一切商品对它们的所有者是非使用价值,对它们的非所有者是使用价值。”对这句话应理解为( )。
A、商品不可能既有使用价值又有价值
B、商品所有者同时获得使用价值和价值
C、使用价值是商品的本质属性
D、商品的使用价值和价值是相互排斥的
If you read this book, you will have a better _____ of the causes of trouble.
A、understanding
B、learning
C、knowledge
D、explanation