Question

Karate player break the brick in a single blow . Why?

Answer 1

Because a karate player strikes the brick or pile of tiles or slab of ice with his hand very fast. So, large momentum of his hand is reduced to zero in a short interval of time which exerts a very large force on the brick to break it.
This large force, we can understand from second law of motion, which says rate of change of momentum is equal to force applied, thus in this case rate of change of momentum on karate player hand is very high thus a huge force is applied on his hand and similar force in opposite direction acts on brick to which breaks it.