Why is the end of the plasma Universe so important to the creation of greater complexity?

not clear

d.) circle motion requires a force to give it a centripetal acceleration.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.