Inertiaalisissa

Inertiaalisissa is a Finnish term that translates to "inertial" in English. In physics, it refers to reference frames where Newton's laws of motion hold true. Specifically, an inertial frame of reference is one in which an object at rest stays at rest, and an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This concept is fundamental to classical mechanics.

There are infinitely many inertial frames, and they are all in uniform motion relative to each other. For example, a stationary observer on Earth is approximately in an inertial frame, as is an observer moving at a constant velocity in a spaceship far from any gravitational influences. The key characteristic is the absence of acceleration in the frame itself.

Non-inertial frames, in contrast, are those that are accelerating. In such frames, fictitious forces (also known as inertial forces) appear to act on objects. These are not real forces but rather a consequence of the observer's acceleration. Examples include a rotating turntable or a car accelerating forward. To describe motion in a non-inertial frame, one must either account for these fictitious forces or transform the motion to an equivalent inertial frame. The principle of relativity, as formulated by Galileo and later refined by Einstein, states that the laws of physics are the same in all inertial frames.