🎡 A ball on a vertical circle — energy sets the speed, gravity sets the limit

Energy conservation fixes the ball's speed at every angle. The string tension T is largest at the bottom and smallest at the top; a full loop needs v_top ≥ √(gR).
Step 1 — the ball is launched from the bottom.
θ = 0° · v = 0 m/s · T = 0 N
vtop = 0 · √(gR) = 0 m/s · —
T = m v²/R + m g cosθ
loop needs vtop ≥ √(gR)