Let's take a quick look at the physics of loops, like the ones you see on amusement park roller coasters. We'll start by considering the physics of driving through a vertical loop at a constant speed. The car's acceleration results from the combination of the car's weight plus the contact and friction forces of the track on the tires. Because the car is going at a constant speed, the acceleration only arises in forms of changes in the direction of motion. Riding through a vertical loop would produce an exhilarating experience. What a rider feels in terms of G forces is the reaction force of the car on the rider. There is a minimum speed required to successfully traverse a circular loop as depicted. Below this minimum speed, the tires would have to be pulled by the track as the car approaches the top of the loop, which is impossible. Instead the car becomes a projectile until it bounces off the track below. While our car's inversion of this failure to loop might also look like an exhilarating ride, the impacts after falling nearly 20 meters would not likely be healthy. It turns out that the performance requirements to traverse a vertical loop at constant speed are well beyond most real life cars. Amusement park rides speed up and slow down as they coast through their loops. A roller coaster will gain speed from the hell, lose some of that speed as it climbs the loop, then regain that speed as it exits the loop. This can all be understood in terms of potential and kinetic energy. Because the coaster has to enter the bottom of the loop with a greater speed than a car powering through the loop at constant speed, the G forces experienced by the riders of the coaster are also greater. It turns out that most looping coasters do not use a circular shape. Amusement park loops will generally have a greater radius of curvature at the bottom of the loop where the speed is greatest. This allows the G forces experienced by the riders to be evened out over the course of the loop.