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Escaping from the
Earth{{/B}} The Earth has a force that pulls things toward itself. We call this force gravity(地心引力). This is something we live with all the time, and we take it for granted and hardly ever think about it. But it is a most important factor in rocket operation and must be overcome if we are to get anywhere in space, or off the ground at all. Take the throwing of a hall as an example. The harder the ball is thrown, the faster and higher it will go. What is the secret Its speed. If we could throw the ball hard enough it would go up and up forever and never come down. The speed at which it would have to be thrown to do this is known as escape speed. Of course, we cannot throw a ball hard enough because A. Few spacecrafts complete their flight operations. B. The Earth’s gravity cannot be overcome. C. No future power is needed after the escape speed is reached. D. "Free fall" takes the place of the Earth’s gravity pull. 热门试题:[单选题]干粉灭火器报废年限为()年，维修期限为出厂期满5年和首次维修以后满两年，进行填报维修，每次送修数量不得超过该单元配置灭火器总数量的1/4。
A.10 B.8 C.7 D.6 [单选题]职业道德的稳定性和连续性是( )。
A. 绝对的 B. 相对的 C. 不受当时社会经济关系的制约 D. 不受其他道德原则的影响 [单选题]盲区是超声波探伤系统重要的性能指标之一，因为它关系到（ ）。
A.检出平行于超声波束的缺陷 B.检出位于锻件中心部位的小缺陷 C.检出表面缺陷 D.检出近表面的缺陷 [判断题]食品生产企业应建立食品原料、食品添加剂和食品相关产品的采购、验收、运输和贮存管理制度。( )
A.正确 B.错误 [多选题]“煤矿没有双回路供电系统”重大事故隐患，是指有下列情形之一（ ）。
A.A、单回路供电的 B.B、有两回路电源线路但取自一个区域变电所同一母线段的 C.C、进入二期工程的高瓦斯、煤与瓦斯突出、水文地质类型为复杂和极复杂的建设矿井，以及进入三期工程的其他建设矿井，未形成两回路供电的 [多选题](1687727)我国稠油油藏依照储层类型及其发育状况划分为( )三大类。
A.多层状稠油油藏 B.块状稠油油藏 C.单层状稠油油藏 D.边底水稠油油藏 [单项选择]( )不是钢电弧焊时氢的危害。
A. 产生气孔 B. 产生白点 C. 引起氢脆 D. 引起热裂纹 [填空题]1992年11月,大陆的两岸关系协会与台湾的海峡交流基金会就解决两会事务性商谈中所达成的以口头方式表达的“海峡两岸均坚持一个中国原则”的共识,后被称为“ ”。 （ ）
[单选题]应急管理部黄明书记提出关于“站在对党和人民负责的政治高度，立足（），从严从难从紧建设通信体系”的指示要求。
A.工作实际 B.地区差异 C.常见问题 D.极端条件 [单项选择]
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