题目

   Winter begins in the north on December 22nd.People and animals have been doing  what they always do to prepare for the colder months, Sqzeirrels,  for example,  have  been busy gathering nuts from trees.    3 6       They examined differences between red squirrels and gray squirrels in the American  state of Indiana.  The scientists wanted to know how these differences could affect the  growth of black walnut(黑胡桃）trees.37    The black walnut tree is also a central  part of some hardwood forests.       Rob Swihart of Purdue University did the studＡ、Ｂ两小球同时从距地面高为h＝15m处的同一点抛出，初速度大小均为v0＝10m/s.Ａ球竖直向下抛出，Ｂ球水平抛出，（空气阻力不计，g取10m/s2）.求： （1）Ａ球经多长时间落地？ （2）Ａ球落地时，Ａ、Ｂ两球间的距离是多少？