题目

Imagine having an ear made out of an apple. It seems like a crazy idea. But it could happen in the near future.Canadian biophysicist Andrew Pelling used an apple to grow a human ear. He thinks fruit and vegetables can be used to cheaply repair human body parts in the future.Scientists have been trying to grow organs in labs to replace our old ones. But it‘s a hard job. In the past, scientists have used man-made materials, animal parts and even dead people as scaffolds（支架）. But that has proved to be difficult and expensive. Pelling and his team, however, found the apple to be a cheap 相距20m的两个小球A和B沿同一直线同时同向向右运动，B球以-2.5m/加速度做匀减速运动，A球以2m/s速度做匀速运动如图所示．求B球初速度多大时，B球恰追上A球．(小球半径忽略不计)