问题
选择题
如图所示，沿竖直杆以速度υ匀速下滑的物体A通过轻质细绳拉光滑水平面上的物体B，某一时刻，当细绳与竖直杆间的夹角为θ时，物体B的速度为（ ）
A．
B．
C．υ
D．
答案
答案：B
A的实际速度向下，则合速度向下，两个分速度分别沿绳子的方向和垂直绳子的方向，B的速度大小与沿绳子的分速度相等，为vcosθ
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如图所示，沿竖直杆以速度υ匀速下滑的物体A通过轻质细绳拉光滑水平面上的物体B，某一时刻，当细绳与竖直杆间的夹角为θ时，物体B的速度为（ ）
A．
B．
C．υ
D．
答案：B
A的实际速度向下，则合速度向下，两个分速度分别沿绳子的方向和垂直绳子的方向，B的速度大小与沿绳子的分速度相等，为vcosθ
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| In some science fiction movies, the robots are just like humans. They help with the housework and do the most 1 jobs. Some scientists believe that there will be such robots in the future. 2 , they agree it may 3 hundreds of years. Scientists are now trying to make robots 4 people and do the same things as us. But robot scientist James White 5 . He thinks that it will be 6 for a robot to do the same things as a person. 7 , it's easy for a child to wake up and know where they are. Mr White thinks that robots won't be able to do this. But other scientists disagree. They think that robots will be able to talk to people 8 25 to 50 years. Robot scientists are not just trying to make robots look like people. For example, there are already robots 9 in factories. They do simple jobs over and over again. People would not like to do such jobs and would get bored. But robots will 10 get bored. In the future, there will be more robots everywhere, and humans will have 11 work to do. New robots will have many different 12 . Some will look like humans, and 13 might look like snakes. After an earthquake (地震), a snake robot could help look for people under buildings. That may not seem possible now, but computers, space rockets and even electric toothbrushes 14 impossible a hundred years ago. We never know 15 will happen in the future! | ||||
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