Question

The weak nuclear forces appear only in certain nuclear processes. Explain

Answer 1

The weak nuclear force is one of the four fundamental forces that govern all matter in the universe (the other three are gravity, electromagnetism and the strong force). While the other forces hold things together, the weak force plays a greater role in things falling apart, or decaying. 
The weak nuclear forces appears only in certain nuclear processes such as beta  decay of a nucleus. In beta decay, nucleus emits an electron and an uncharged particle called neutrino. It is not as weak as the gravitational force, but much weaker than strong nuclear and electromagnetic forces.The range of weak nuclear force is exceedingly small, of the order of 10^(-16) m.