Hi Babu,
Have a look at this site:
http://www.mvps.org/project/networkanalysis.htm
When you've followed it through, have a look at the final network diagram
which shows the ciritical path. If, however, the Machine task, which has a
duration of 3 and a slack of 8, had a duration of 11, it would have no slack
and thus also be critical. Thus the critical path would go through the
Machine task as well.
For another example, if we were to add a project management task which
lasted the whole project's duration, it would also be critical.
We would now have 3 critical paths
Hop this helps,
Mike Glen
Project MVP