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.