knock on与knock at的区别是什么？

knock on与knock at的区别是什么？

knock on 敲(门裤猛拿、窗等);撞knock at 敲(门,窗等)Knock on the door before you enter.进屋之前要胡搭先敲门。When I fell I got a terrible knock on the head.我摔倒时头撞得很厉害。He got a nasty knock on the head.他头部遭到重重一击。Did I hear a knock at the door?是知饥有人敲门吗?I was wakened by a knock at the door.敲门声把我吵醒了