So I've always been confused about this. Suppose you have your normal pendulum: length L, mass m, and angle Θ. When you describe the potential energy PE = mgh, you must decide where to measure your h from. Throughout my years I've seen it measured from the mass to the 0 equilbrium point where you'd get that PE = mgL*(1-cosΘ) and also measured from the mass to the horizontal position where Θ=π/2 where you would get PE = -mgLcosΘ. the signs are with respect to the positive y-axis pointing up. These are clearly not the same number, so whats the distinction? what is the actual potential energy? why have i seen it done both ways?