For collinear collision, the preceding collision velocity and post collision velocity are calculated on the basis of the slippage distance and unreturnable distortion of collision vehicle;

对于一维碰撞，根据碰撞后车辆的滑移距离和车辆的塑性变形量，反推计算出车辆碰撞后的速度和碰撞前的接触速度；

youdao

She had 13 aces and 30-40 unreturnable serves against Zheng Jie in the semi-final.

BBC: Tracy Austin on Venus v Serena

But Clijsters upped the intensity in game eight to claim the decisive break and followed it up by racing to 40-0 and converting her first match point with an unreturnable serve.

BBC: Wimbledon 2010: Venus Williams wins opening match