1. The problem statement, all variables and given/known data A small bead is threaded on a smooth circular wire of radius r which is fixed in a vertical plane. The bead is projected from the lowest point of the wire with speed √6gr. FInd the speed of the bead when it has turned through: (a) 60°, (b) 90°, (c) 180°, (d) 300°. Answers: (a) √5gr, (b) √4gr, (c) √2gr, (d) √5gr 2. The attempt at a solution While this question is explained here, I do not understand one part. Why PE = m * g * h = m * g * (r - r * cos(θ)). Specifically h = r - r * cos(θ)? Why isn't it equal to just h = r cos 60°? Taking this image (as I see the problem): We have φ = 60°. R is h and equals to r cos 60°, where r is the adjacent line to the angle. Why the correct height h is r - r cos 60° and not just r cos 60°?