问题
选择题
人造地球卫星在圆形轨道上环绕地球运转，它的运动速度、周期和轨道半径的关系是
A．半径越大，则速度越大
B．半径越大，则速度越小
C．半径越大，则周期越大
D．半径越大，则周期越小
答案
BC
由线速度公式
可知A错；B对；由周期公式
可知半径越大周期越大，C对；D错；
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人造地球卫星在圆形轨道上环绕地球运转，它的运动速度、周期和轨道半径的关系是
A．半径越大，则速度越大
B．半径越大，则速度越小
C．半径越大，则周期越大
D．半径越大，则周期越小
BC
由线速度公式可知A错；B对；由周期公式可知半径越大周期越大，C对；D错；
| 阅读短文，从各题所给的四个选项中，选出一个正确答案。 | ||||
| All over the world people swim for fun. Swimming is enjoyed by people of 1 ages, from the very young to the very old. There are many places for people to swim in swimming pools. Many schools, hotels and clubs have swimming pools. Some people have pools of 2 in their yards. Swimming is one of 3 forms of exercise. It can 4 hearts and bodies strong. It can also help blood circulate (循环). Handicapped people can keep their bodies in better condition by swimming 5 they can't enjoy sports. 6 is a good idea for parents to see to it (务必做到) that their children learn to swim 7 an early age. So their children will 8 it for the rest of their lives. There are many rules for water safety. These rules can help save not only your life 9 the life of a friend. First of all, know 10 . Many schools 11 swimming lessons to children. Adults (成人) can learn to swim at public pools. 12 rule to remember is never to swim 13 . Always swim with a friend and know 14 in the water at all times. It is best to swim 15 in safe places if you are beginners. If every one learned to swim and obeyed the rules for water safety, most drowning (溺水) could be avoided. | ||||
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