The pulling of objects toward Earth by gravity is called G force.   G forces on a roller coaster ride increase when the train goes down the hill.   The opposite happens when the train goes up the hill.  G forces affect your stomach, ears, and joints.  As the forces increase, your stomach will start feeling squashed.  G forces are constantly increasing and decreasing during a roller coaster ride.

A roller coaster follows many scientific rules, such as the rules for gravity and momentum. Another force that acts on a roller coaster during its ride is called inertia.  Inertia keeps an object moving in a straight line unless a different force stops it from doing that.  The track uses inertia to control the train.  If centripetal force didn’t exist in a loop, inertia might keep a train moving in a straight line right off the track.  Fortunately inertia and the track both do their jobs.