Why Always report errors

[glv]

Hello @jiajia,

This was answered here:

This is the third chapter of the book "The Nature of Code" about friction and resistance, I don't know what the problem is, as a beginner this is my first problem I can't solve, so I ask you for help.THANKS！

Hello, The answer is here: https://natureofcode.com/book/chapter-2-forces/ The chapter tells you which class to put the functions in.