How Do You Derive the Equation of Motion for an Inverted Pendulum with Springs?

[2slowtogofast]

Can someone explain how they got the term with the red box
I understand the kl/2 but not the sin and cos part

**Question** Consider the inverted pedulum connected to two equal springs both with constant k. Springs are undelfected when the mass is in the vertical position. If the rod is of length l and the spring are connected at l/2 find the equation of motion.

http://img442.imageshack.us/img442/4896/24111q.jpg

 

[PeterO]

Can someone explain how they got the term with the red box
I understand the kl/2 but not the sin and cos part
http://img442.imageshack.us/img442/4896/24111q.jpg

If that picture represents the total problem as presented, then there is no solution, as no question is asked?

 

[2slowtogofast]

Sorry I was on my phone and forgot the question I edited my orginal post