Answer: Models.
Explanation: Models have usually used to simplification the harder subjects, and make it easier to work with.
An example of this is when you do the math for a car moving in the street, where you suppose that the car has an easy shape, like a box, you ignore things like air resistance and such.
Models are used in almost every field of physics, especially in the experimental field, where having a lot of variables is a problem, so the scientist develops models where some of the variables can be ignored, and in this way, the experiment and the math are easier.
