题目

We all need to exercise．Doctors say it is good for us．It makes our heart and body strong．It also gives you more energy(能量)．And you will feel better about yourself．It’s good to exercise twice a week．Twenty minutes each time is enough．There are many ways to exercise．You can walk，run，play sports or swim．Many people go to special places to exercise．They are called “fitness centers(健康中心)”．These places have a lot of equipments(设备)．People can do different sports there．Some people buy equipments for their homes．But it is very expensive．Exercising发射地球同步通信卫星的基本方法是，先用火箭将卫星送入一近地椭圆轨道a运行，然后开动星载火箭变轨，将其送入与地球自转同步的轨道b，变轨点选在轨道a与轨道b内切的Q点，如图所示。下列说法正确的是A．卫星在Q点变轨时加速度变小B．卫星变轨后在Q点的机械能比变轨前的大C．卫星在轨道b运行速度小于在轨道a运行时的最小速度D．卫星在Q点变轨时星载火箭喷射气体的方向与卫星线速度方向相同