Guess what? Gravity is also a force and it acts upon us every day, only we don't usually notice. Check out the video for the Giant Drop, a theme park ride at Dreamworld on the Gold Coast of Australia.
So how does this relate to roller coasters? Well it's like a vertical roller coaster and it helps to demonstrate how gravity exerts a force on people (and everything else) just like acceleration.
Force = mass x acceleration
Force of gravity = mass x acceleration due to gravity
On earth, the acceleration due to gravity is 9.8 meters per second per second downwards.