I am confused about how the principle of conservation of energy can be used to predict the velocity of a pendulums for any given height and angle. For example, say I use the equation U = -K (potential is equal to kinetic) to solve for the velocity at 15 degrees of a pendulum bob whose wire length is 1.2 m. The answer turns out to be 1.5 m/s. But this confuses me. The bob is not in free fall because there is the tension of the wire pulling on the pendulum bob. So what does this 1.5 m/s refer to? How does the equation "know" that the bob is swinging in an arc and not vertically falling?