Conical Pendulum | Physics Motion Laws

[nokia8650]

http://img25.imageshack.us/my.php?image=77494342.jpg

I can do the first part, however am unsure of how to deduce the motion cannot take place unless the inequality is satisfied. Can someone please explain this part?

Thanks

 

[Office_Shredder]

Assume the opposite inequality holds, and plug it into the equality you got (giving you an inequality) and see if a contradiction pops up

 

[nokia8650]

ahh yes, i see, thank you!