Newton's first law holds in:

### Understanding Newton's First Law Newton's First Law of Motion, often called the **Law of Inertia**, states that an object will remain at rest or move with a constant velocity unless acted upon by an external net force ($F_{net} = 0$). ### Reference Frames To apply Newton's laws, we must consider the **frame of reference** from which an observer is viewing the motion: 1. **Inertial Frames**: These are frames of reference that are either at rest or moving at a constant velocity (zero acceleration). In these frames, Newton's laws hold true exactly as stated. 2. **Non-Inertial Frames**: These are frames that are accelerating or rotating. In these frames, objects appear to accelerate even when no physical force is applied (due to inertia). For example, if you are in a car that suddenly brakes, you feel pushed forward even though no one touched you. To make Newton's laws work here, we have to invent "fictitious" or "pseudo-forces." ### Conclusion By definition, an **inertial frame** is a frame in which Newton's First Law holds. Therefore, it does not hold in accelerating or rotating (non-inertial) frames without the addition of pseudo-forces. Final Answer: **Only inertial frames**.