two forces on the rock
tension in the rope and gravity
What is the force diagram for a rock that's tied to a rope and is swinging?
2 answers
a quick check with google turned up this helpful article.
www.khanacademy.org/computing/computer-programming/programming-natural-simulations/programming-oscillations/a/trig-and-forces-the-pendulum
www.khanacademy.org/computing/computer-programming/programming-natural-simulations/programming-oscillations/a/trig-and-forces-the-pendulum