搜题请搜索小程序“爱下问搜题”
问题 选择题

下列说法中正确的是（　　）

A．匀速圆周运动是一种匀速运动

B．匀速圆周运动是一种匀变速曲线运动

C．匀速圆周运动是一种平衡状态

D．平抛运动是一种匀变速曲线运动

答案

A、匀速圆周运动速度大小不变，方向变化，速度是变化的，是变速运动，受力不平衡，故AC错误；

B、匀速圆周运动加速度大小不变，方向始终指向圆心，加速度是变化的，是变加速运动，故B错误；

D、平抛运动只受重力，加速度为g，是匀变速曲线运动，故D正确．

故选D

选择题

既是春秋五霸又是战国七雄的诸侯国是　

①齐　②燕　③楚　④赵　[ ]

A、①②　　　　　

B、②③　　　　　　

C、③④　　　　　　

D、①③
查看答案
单项选择题

The great advance (91) rocket theory 40 years ago showed that liquid-fuel rockets were far (92) in every respect to the skyrocket with its weak solid fuel, the only kind of rocket then known. (93) , during the last decade, large solid-fuel rockets with (94) fuels about as powerful as liquid fuels have (95) their appearance, and it is a favorite layperson’s question to (96) which one is "better". The question is meaningless. (97) might as well ask whether a gasoline or a diesel engine is "better". It all (98) on the purpose. A liquid-fuel rocket is complicated, (99) has the advantage that it can be controlled beautifully. The burning of the rocket engine can be stopped completely; it can be reignited when (100) . (101) , the thrust can be made to vary by adjusting the speed of the fuel pumps. A solid-fuel rocket, on the other hand, is rather (102) in construction, (103) hard to build when a really (104) size is desired. But once you have a solid-fuel rocket, it is ready for action (105) very short notice. A liquid-fuel rocket has to be fueled first and cannot be (106) readiness for very long after it has been fueled. (107) , once a solid-fuel rocket has been ignited, it will keep burning. It cannot be stopped and reignited (108) desired (it could conceivably be stopped and reignited after a pre-calculated time of burning has elapsed, and its thrust cannot be varied. Because a solid-fuel rocket can be kept ready for a long time, most military missiles (109) solid fuels, but human-piloted spaceflight needs the fine adjustments that can (110) be provided by liquid fuels.

A．The person

B．One

C．The other

D．It
查看答案