Riding in a roller coaster is a lot like riding in a car. Think about when the driver takes a turn a little fast. As the car is turning to the left, you can feel your body move to the right. In fact, if you weren't buckled in, you'd probably get tossed to the other side of the car.

Taking the loop-the-loop in a roller coaster is a lot like taking a fast turn in a car. As you approach the loop, your inertial velocity is straight ahead of you. This means that your body wants to keep moving straight because up to this point you and the roller coaster car were moving straight. But the track keeps the coaster car, and therefore your body, from traveling along this straight path. The force of your acceleration pushes you from the coaster-car seat, and your inertia pushes you into the seat. Your own outward inertia creates a sort of false gravity that stays fixed at the bottom of the coaster seat even when you're upside down. You need a safety harness for security, but in most loop-the-loops, you would stay in the coaster-car seat whether you had a harness or not.

As you move around the loop, the net force acting on your body is constantly changing. At the very bottom of the loop, the acceleration force is pushing you down in the same direction as gravity. Since both forces push you in the same direction, you feel especially heavy at this point. As you move straight up the loop, gravity is pulling you into your seat while the acceleration force is pushing you into the floor of the car. You feel the gravity pulling you into your seat, but (if your eyes are still open) you can see that the ground is no longer where it should be.

At the top of the loop, when you're completely upside down, gravity is pulling you out of your seat, toward the ground, but the stronger acceleration force is pushing you into your seat, toward the sky. Since the two forces pushing you in opposite directions are nearly equal, your body feels very light. As in the sharp descent, you are almost weightless for the brief moment when you are at the top of the loop. As you come out of the loop and level out, you become heavy again.

The loop-the-loop is amazing because it crams so much into such a short length of track. The varying forces put your body through the whole range of sensations in a matter of seconds. While these forces are shaking up all the parts of your body, your eyes see the entire world flip upside down. To many coaster riders, this moment at the top of the loop, when you're light as a feather and all you can see is the sky, is the best part of the whole ride.