Physics: Why Minimum Speed Keeps a Looping Cart on Track

At the top of a vertical loop, a cart is pulled toward the centre by two forces: its weight, mg, acting downward, and the normal reaction from the track, N, also directed downward. Together these supply the centripetal force keeping the cart moving in a circle, so mg + N = mv²/r. The balance between them is what determines whether the cart stays pressed against the track or loses contact entirely. This matters because the minimum-speed condition, where N drops to zero and weight alone provides the required centripetal force, sets a hard limit on how slow the cart can safely travel. As the cart moves from bottom to top, N swings from a maximum, where passengers feel heaviest, down toward zero, where they feel weightless. Understanding this variation explains both the thrill and the safety engineering behind loop design.