I don't understand your question. If the frame was inside the car it would cease to be an inertial frame. The question is to solve the problem in an inertial frame. Not necessarily any particular inertial frame. Given this, I was simply assuming in the quote above that an inertial frame that...
Sorry. It should be "inertial frame". In my textbook the solution is given for a non-inertial frame. The challenge was posed to us to try to solve this problem within an inertial frame. Thanks for letting me know about that typo. I will edit it right away.
Homework Statement
A small weight of mass 'm' hangs from a string in an automobile. Initially, the car is at rest with the weight hanging vertically. Then, the car SUDDENLY accelerates to a rate 'A'. Find the maximum angle \varphi_{max} through which the weight swings by analyzing the motion...