In an accelerated frame like a car you would see objects accelerating under no external force. That's because General relativity tells us Newton's 2nd law is actually more like

$$F=ma+(\text{curvature})$$

Where the curvature term includes gravity and accelerated reference frames. Newton's law is valid when the 2nd term is negligible (approximately zero curvature).

EDIT: A bit more detail. You correctly mention that Newton's law is valid in non-accelerated frames. The first term encompasses the accelerated motion (change in momentum) of the object in the frame. The second term accounts for the acceleration of the frame itself. The whole thing is called a covariant derivative if you want to know. But formula we know as Newton's 2nd law is the case where the 2nd term is zero, which only happens if the frame's acceleration is zero, i.e. the frame is inertial.

In an accelerated frame like a car you would see objects accelerating under no external force. That's because General relativity tells us Newton's 2nd law is actually more like

$$F=ma+(\text{curvature})$$

Where the curvature term includes gravity and accelerated reference frames. Newton's law is valid when the 2nd term is negligible (approximately zero curvature).

EDIT: A bit more detail. You correctly mention that Newton's law is valid in non-accelerated frames. The first term encompasses the accelerated motion (change in momentum) of the object in the frame. The second term accounts for the acceleration of the frame itself. The whole thing is called a covariant derivative if you want to know. But formula we know as Newton's 2nd law is the case where the 2nd term is zero, which only happens if the frame's acceleration is zero, i.e. the frame is inertial.

EDIT 2: Note, I wasn't entirely happy with the word "curvature" for the 2nd term, but thought it sufficed for the conceptual purpose. Perhaps more accurate would be:

$$\frac{F}{m}=a_{\text {body}}+a_{\text {frame basis}}$$

In an accelerated frame like a car you would see objects accelerating under no external force. That's because General relativity tells us Newton's 2nd law is actually more like

$$F=ma+(\text{curvature})$$

Where the curvature term includes gravity and accelerated reference frames. Newton's law is valid when the 2nd term is negligible (approximately zero curvature).

That's a very loose conceptual description, But hopefully helps you.

In an accelerated frame like a car you would see objects accelerating under no external force. That's because General relativity tells us Newton's 2nd law is actually more like

$$F=ma+(\text{curvature})$$

Where the curvature term includes gravity and accelerated reference frames. Newton's law is valid when the 2nd term is negligible (approximately zero curvature).

EDIT: A bit more detail. You correctly mention that Newton's law is valid in non-accelerated frames. The first term encompasses the accelerated motion (change in momentum) of the object in the frame. The second term accounts for the acceleration of the frame itself. The whole thing is called a covariant derivative if you want to know. But formula we know as Newton's 2nd law is the case where the 2nd term is zero, which only happens if the frame's acceleration is zero, i.e. the frame is inertial.