题目

“Four eleven-year-old boys, Wow!”Standing sleepily in the store's entrance, I_______this strange sight. My dad had just dropped me off to shop for Christmas _______ but my brother and his friends had a more noble_______ for shopping.“Do you think Caitlin will like this? one of the boys asked. And my brother, looking rather _______, was standing in front of a shelf of socks. Okay. The pink one would be a good idea,” I _______.He picked up one and presented it to me for_______.They used up all their _______some of which they had saved up over the course of the year for Christmas gifts. B17．在倾角为30°的光滑斜面底端固定一挡板，质量均为m的物块B、C通过轻弹簧连接，且均处于静止状态，此时弹簧的压缩量为x0，O点为弹簧的原长位置．在斜面上距物块B的距离为3x0的P点由静止释放一质量为m的物块A，A与B相碰（不粘连）后立即一起沿斜面向下运动，并恰好能返回到O点．物块A、B、C均可视为质点，重力加速度为g．（1）求A、B碰前弹簧具有的弹性势能；（2）若物块A从P点以一定的初速度沿斜面下滑，两物块A、B返回O点时还具有沿斜面向上的速度，此后恰好可以使C离开挡板而不上滑，求物块A在P点的初速度v0．