Problem in understanding inertial and non-inertial frame of reference

Consider a person standing still on the ground and a car, which is moving with a constant speed $$v$$ in a straight line. From the frame of reference of the car the person is moving backwards with same speed $$v$$.

What causes to the person to move backwards in the frame of reference of the car?

In the case of an accelerating frame of reference we can assign a pseudo force but in this case the acceleration of our frame of reference (i.e. the car) is zero, so we can't say that pseudo force is acting on the person on the ground.

• This is just geometry based on relative motions of different reference frames. No force or pseudo-force necessary.
– jpf
Commented Jan 15, 2021 at 12:54
• Probably not a bad idea to change the title to something a bit more descriptive. It's impossible to tell what the question is about from a glance. Commented Jan 15, 2021 at 14:16

As you said, in the reference frame of the car, the person is moving at a constant speed $$-v$$. Assuming that $$|v|$$ is much less than the speed of light, we may apply Newtonian mechanics in this situation. The first Newton's law states that a body free of forces is at rest or moving in straight lines at constants speeds, as seen by an inertial reference frame. So this is exactly the case of the person moving in respect to the car (which is an inertial frame): It is free of any forces since it is moving at a constant speed!