What keeps you in your seat on an upside down roller coaster?

For restless people looking to get their adrenaline pumping, there are always roller coasters to do the trick. And every time you get on a roller coaster, there’s always an attendant to make sure your seat belt or seat bar is on correctly. However, when the roller coaster spins upside down, it’s not these devices that keep you in place, but physics.

Basically, acceleration is a change in direction or speed. But when something moves in a circle, it is constantly changing direction, therefore it is constantly accelerating, even if its speed remains the same. The force that causes acceleration toward the center of a circle is called the centripetal force. If you tie an object to a string and then swing it in a circle, the string remains taut and your pull forces the object to move in circles. You can feel the constant force on the other end of the string, but if you let go, the object goes flying.

When you ride a roller coaster through a loop, a similar process takes place. You are constantly changing direction, therefore accelerating, as the track is constantly pushing the car in a new direction. The car pushes against the track and the track pushes against the car and as a result the car seat pushes you in the same direction.

You get used to an upward push against the downward pull of gravity, so when you get to the end of the loop, you don’t feel the upward push. As it moves through the loop, its own momentum tends to keep it moving in a straight line, away from the center of the circle. At the same time, the centripetal force always pushes you towards the center of the circle. When you go over it, you feel like you have to stay airborne, but the car pushes you to avoid it. You would stay in the car even without the seat bar. However, don’t try it.

Then, when you go through the bottom of a pothole or over the top of a hill on the roller coaster, the centripetal force will affect you again. As the roller coaster goes down a decline, the centripetal force will constantly push it up toward the center of the curve, while the rider is pulled down by gravity. You feel heavier in the depths due to the combination of the upward force and gravity. As you drive over the crest of a hill, centripetal force pulls the car below you toward the center of the curvature. But this time, the car doesn’t push you because you are on top of it and the force is downwards. Your momentum carries you up until the seat bar exerts a downward force. At this point, the security bar becomes extremely important. If the force is great enough, you will feel weightless at the point where the downward force of gravity is exactly equal to your upward inertia.