Describe a system in which the main forces acting are parallel or antiparallel to the center of mass, and justify your answer.
A parachuter or a car driving, could be examples.
This is College Physics Answers with Shaun Dychko. A parachuter falling is an example of a system where you have forces that are parallel and anti-parallel. You have gravity down, you have friction upwards, or you might have a system consisting of a car and you'd have the car accelerating and exerting some force to propel itself and you'd also have some drag force going the other way.