Why are the loops on a roller coaster tear-drop shaped instead of circular? it would be simpler to design & build tracks with a circular shape, so why do roller coaster manufacturers use a tear-drop shape instead? be specific, and include a discussion of the forces acting on the rider, their centripetal acceleration, etc. as needed to make your point.

The centripetal force acting on the rider is much greater if the roller coaster has a circular loop, rather than oval. This is because the change in direction is much sharper throughout the loop, causing the rider to experience a much more intense G-Force throughout the loop. 
A teardrop loop features a more gradual change of direction: the cart spends time moving upward, briefly changes direction, and spends the rest of the time moving downward and flattening to a horizontal path. This means the riders experience the majority of the force on the way down as the car levels out, rather than an intense G-force throughout the ride.