This is a standard fuzzy dice problem, hung in a car, at angle, theta, and I have to find the acceleration of the car. I understand the approach used by the prof where he sets the x-y plane relative to mg. So the sum of forces in the x direction = ma = Tsin(theta) Similarly, in the y direction, Tcos(theta) = mg; so a = gtan(theta). However, when I was going over my notes, I was wondering why can't the plane be set with respect to T and split the mg into its components. Therefore T = mgcos(theta) and ma = mgsing(theta). and obtaining a = gsin(theta). Why can't this approach be used to solve for the acceleration of the car.