Reference frame

Think of a frame as a set of axes with an observer riding along on them. AP Physics 1 EK 1.4.A.1 states that the choice of frame determines the direction and magnitude of the quantities that observer measures, so a velocity quoted with no frame attached is an incomplete answer.

An inertial frame is one in which Newton's first law holds. The CED defines it that way in EK 2.4.A.5: a frame from which an observer would verify the first law of motion. In practice that means a frame at rest or moving at constant velocity. Any frame accelerating relative to an inertial one is noninertial, and inside it objects appear to accelerate with no second object to blame.

That is the distinction worth marks, because $\vec{F}_{net} = m\vec{a}$ in its plain form is only valid in an inertial frame. A ball on the dashboard of a braking car slides forward relative to the car with nothing pushing it. The fault is in the frame, not in the physics.

Inertial observers disagree about velocity and agree about acceleration, which EK 1.4.B.2.ii states directly, so they also agree about forces. An AP Physics 1 boundary statement lets you assume any frame is inertial unless a problem says otherwise.

Converting a measurement from one frame to another is relative velocity. Topic 1.4 Reference Frames and Relative Motion carries the AP treatment.