In order to calculate the tension in the pendulum cod, let's start by sketching the problem:
At this point, all the mechanical energy is potential energy, given by:
At position 20°, we will have both kinetic and potential energies:
Now, let's decompose the weight force in the directions along (Wy) and perpendicular (Wx) to the cord:
The sum of the forces along the cord will be equal to the centripetal force:
From the kinetic energy equation, we have:
Using this value of v² in the previous equation, we have: