Energy conservation in simple pendulum

[imjudit]

Hello everyone! I've found this problem in my exercises book, and I'm having slight troubles solving it.
"Consider a simple pendulum with mass m, on a string with l length. Considering that the position of pendulum is determined with the angle $\theta$ and noting that the string DOES NOT do any work, considering all the forces acting on the pendulum show that the energy conservation law is applied"

Well, I know how to prove that the ECL is applied without using the forces (considering that the Ep is 0 at the lowest point, and that the Ek is 0 at the highest).
Any ideas?

Thanks in advance

(I wasn't sure if this is supposed to go to "homework" section, since it isn't a homework...)

 

[kuruman]

The (so called) law of mechanical energy conservation is the work-energy theorem in disguise in the case when only conservative act on the system to which one may add non-conservative forces that do no work, as is the case here. The work energy theorem says$$\Delta K=W_{net}$$Just find expressions for the work done by gravity $W_g$ and the change in kinetic energy $\Delta K$. Remember that the change in gravitational potential energy is the negative of the work done by gravity.