Knock on 和Knock at 有什么区别啊?

Knock on:敲（门、窗等）例句与用法:1。He accidentally knocked on (the pass from Jones)。他非故意犯规,用手向前击了（从琼斯传来的）球。2。Jim sat up when he heard the knock on his bedroom door。吉姆听到敲寝室门的声音时便坐了起来。3。Please do not enter before knocking on the door。进来前请先敲门。Knock at:vt。敲例句与用法:1。There was a furious knock at the door。有人在猛烈地敲门。2。Did I hear a knock at the door?是有人敲门吗?3。Did you hear the knocks at the door?你听到敲门声了吗?没什么大区别