To hit with something heavy, or bang with your fist.
The runner knocked every hurdle down as he tried to jump over them.
Even though the K at the beginning of the word is silent, the C in the CK at the end is, too.
To crouch down on your knees.
I had to kneel down to reach under the desk.
Not only does this word have a silent K, its past tense can either be 'knelt' or 'kneeled.'