knocked
v.v. to hit a hard surface with your hand or an object to get someone's attention. You usually do this to a door before you enter a room.
v. to strike a surface with a sharp blow, typically to attract attention or gain entry. Transitive when used with a direct object, or intransitive when followed by a prepositional phrase.
She knocked on the door before entering the office.
The delivery driver knocked loudly on the front door because the doorbell was broken.
The wind was so strong that it knocked the heavy flowerpots off the balcony and onto the pavement below.
Often used intransitively with the preposition 'on' when referring to doors, or transitively when describing an object being hit or moved.
I knocked the doorI knocked on the doorWhen the goal is to get someone's attention, use the preposition 'on'. Without 'on', it implies you are physically hitting or damaging the door.