This is the 2909th most frequent Polish word.
mocnym
The Polish word "mocnym" means "strong" in English (instrumental or locative singular form of "mocny").
Here, 'mocnym' is an adjective describing 'kandydatem' (candidate) in the instrumental case, indicating strength or capability.
Ten samochód był mocnym kandydatem, by wygrać wyścig.
This car was a strong candidate to win the race.
In this sentence, 'mocnym' is an adjective describing 'wstrząsie' (shock/shake) in the instrumental case, referring to its intensity.
Po mocnym wstrząsie, woda przestała być mętna.
After a strong shake, the water stopped being murky.
'Mocnym' here is used to describe 'zawodnikiem' (player), showing strength or prowess in a specific skill.