Many people like pandas very much. They are very 1   . Most of them live in Sichuan,    2  .  They    3 white hair,  but their shoulders,  legs,  ears and eyes are    4  . Pandas are usually heavy,  but they can climb trees like  5   animals.
   Pandas’ favorite food is bamboo    6  . But long long ago(很久很久以前),  pandas liked to eat meat. It takes pandas a lot of time to have food every day. When pandas are full(吃饱的),  they like    7 . They relax many hours every day.
   However, there are   8   pandas in the world today.    9   some people cut down (砍伐) too many bamboo trees and pandas can not find enough(足够的) food.
   I think we must    10   them

$\widehat{AB}$是竖直平面内的四分之一圆弧轨道，在下端B与水平直轨道相切，如图所示．一小球自A点起由静止开始沿轨道下滑．已知圆轨道半径为R，小球的质量为m，不计各处摩擦．求