火箭发射“神舟”号宇宙飞船开始阶段是竖直升空,设向上的加速度a=5m/s2,宇宙飞船中用弹簧秤悬挂一个质量为m=9kg的物体,当飞船升到某高度时,弹簧秤示数为85N,那么此时飞船距地面的高度是多少(地球半径R=6400km,地球表面重力加速度g=10m/s2)?
解:升空到距地h高处,对飞船中的物体进行受力分析,运用牛顿第二定律列出等式:
F﹣mg'=ma,
又由于万有引力近似等于物体重力得:
开始升空时:mg= GMmR2
A. What about you?
B. By bus, but sometimes I ride to school.
C. I live far from school.
D. I often go to school by bus.
E. How far is it?
A: Where do you live?
B: ____
A: ____
B: I live 10 kilometers from school.
A: How do you usually get to school?
A: How long does it take?
B: 25 minutes by bus and about 50 minutes by bike.____
A: I live near our school.
B: Do you usually go to school on a bike?
A: No. ____
按课文《春到梅花山》,根据量词,写词语。
一朵 ____ 一条 ____ 一缕____