"Gravitational force and the acceleration force" are the forces that are acting on a passenger while going through a loop.
Explanation:
When the passengers reach the top of the loop, The force of the car's acceleration pushes all the passengers off the seat and towards the center of the loop. On the other hand the inertia will try to push the passengers back into the seat. The gravity as well as the acceleration will tend to push the passenger in opposite direction with approximately equal force, which will create a feeling of being mass-less or we can call it weightlessness.
