Accelerated Frame of Reference
¥You are inside a stationary car on a frictionless floor.
¥What happens if the car begins to accelerate?
Inside the car, it appears that a mysterious force is attracting you to the back wall of the box car. Your frame of reference, the close box car, is accelerating.
Someone outside the car sees the car is accelerating. A force is accelerating the box car, but you are not accelerating. Their frame of reference is not accelerating.