Question

we can easily move our hand in air but to do the same through a solid block of wood we need a karate expert. why?

Answer 1

we can move our hands in the air freely because the molecules are loosely packed whereas in a solid block of wood the molecules are tightly packed

Answer 2

The density of air is very small. The air molecules are separated by a large distance. So very little force / pressure is required to move hands through air.

The molecules or matter of wood is closely packed ie., it is very dense and hard. The force or pressure required to penetrate hands though the wood is very high. The impact or stress has to be very high. So only karate experts can do that.