in Waterfallmodel the requirements must be feasible where in
v-model this is not compulsory.
in waterfall model each and every phase of SDLC should
feasible then only it will go forward.
whereas in V-model SDLC phase all will be work in parallel
method

a)	When the customer or client requirements clear then 
that time use the waterfall model
j)	QA is the verification-developers
k)	QC is the validate-test
l)	UAT is the client


My name is k.chandra sekhar,
My phone no are 9246540049 65880049, Hyderabad.
My mail id is ndra_143@yahoo.co.in 
I am taking the class for projects and interview trips if 
you interest contact

Water fall model when the process is running u cant do any 
kind of changes in the middle of your project.
V-model -- here verification and validation is done 
simoltaneously and the outcome product is a QUALITY product