What is virtual work? And how does it apply to this question?

[Jessielowe123]

Hi All!

I am new on here so I hope you quality people can help me out:)

I have this question and it asks me to solve it using Virtual Work? What is that and how do I apply it to this question:

"A slender rod of length L is attached to a collar at B and rests on a portion of a circular cylinder of radius r. P and Q are two applied point loads. The effect of friction is neglected. For given L, r, P and Q, determine the value of θ corresponding to the equilibrium position of the mechanism by using the method of virtual work."

HERE IS THE ASSOCIATED DIAGRAM:
http://img504.imageshack.us/img504/5771/dsc01399gi8.jpg

After some research I have found the question stems from a book called: Vector Mechanics for Engineers, which I do not own!

Thanks for your time and any help is appreciated :)

 

[jhicks]

The idea of virtual work is that you add a tiny displacement to the system along a movable point (say in this case the piston moves dx to the left as a result of the force Q), how does the position of the rod change? You know that if Q pushes left the piston by dx that this causes the rod's top end to move more vertical and thus does negative work with respect to the downward force P at that point. In order to maintain equilibrium, therefore, P must do positive work equal to this negative work as a result of Q. Your job is to find theta such that this is true for L, r, P and Q from your question.