我孩子學校的 Physics PhD qualifying exam有題挺難的，

北清本科幫 和 俄本 幫 秒殺北美本科幫

有興趣的可試試下麵 link LOL

https://www.chegg.com/homework-help/questions-and-answers/problem-2-80-pts-consider-ballistic-missile-trajectory-suborbital-flight-problem-rocket-bu-q49252024

Not much physics, more algebra in this problem.